Latest News - Richard P. AllanTweets by @rpallanuk
Here is a list of some of my latest news.
Strengthening Walker circulation and the global warming hiatus
New research in Nature Climate Change by England et al. builds upon a number of previous studies implicating the role of the Pacific ocean circulation in explaining the recent slowing of the rise in global average surface temperatures. Using observations and simulations, they demonstrate that an observed strengthening of the Pacific trade winds (which blow from east to west) since the mid-1990s has caused the observed continued heating of the planet (primarily caused by rising atmospheric greenhouse gas concentrations) to be mixed deeper into the ocean rather than warming the surface layers which influence air temperature.
Above: schematic representation of how strengthened atmospheric and oceanic circulations are mixing heat (building up from rising atmospheric concentrations of greenhouse gases) to deeper layers of the Pacific ocean
This new work builds upon previous findings highlighted an increase in sea surface height in the west Pacific caused by strengthening trade winds as part of an intensified Walker circulation across the Pacific with associated upwelling cooling the East Pacific. This region was recently found to be crucial in determining the slowdown in global surface temperature rise.
There is a growing consensus that natural fluctuations in Pacific ocean circulation over many decades (named the Interdecadal Pacific Oscillation or IPO) can contribute positively or negatively to trends in global surface temperature. Climate model simulations can also generate this type of variability in the ocean although are not designed to predict the timing of the lumps and bumps in the temperature record. Previous work has shown that hiatus decades are captured by simulations including increasing greenhouse gas concentrations and that the mechanism is consistent with the England et al. work, namely that heat is increasingly mixed below the ocean surface layers (deeper than 100-300 metres) in the negative phase of the IPO
What is novel about the current work by England et al. is that they construct a simulation which isolates the effect of the increasing wind strength on the mixing of heat into the ocean. Furthermore, they find that while climate models can capture this type of variability, they cannot reproduce the size of the strengthening in trade winds which appear unprecedented in the observational record. What is also intriguing is that these changes are the opposite to the anticipated long term decline in the atmospheric "Walker" circulation in a warming world based upon robust physics and detailed simulations.
An important question, not tackled by the recent study, is: why are these winds increasing? The sea surface patterns generated by the increasing trade winds cause waters to cool in the east Pacific and warm in the west Pacific which in turn can drive further strengthening in the trade winds: a vicious cycle. Therefore it is somewhat of a chicken and egg problem; which is driving which? And are these unprecedented changes associated with rising greenhouse gases or changes in other radiative forcings such as aerosol pollution or solar irradiance? Or are they generated by unforced, chaotic (but slow) fluctuations of the ocean?
The implications or these changes could be substantial. It would be surprising if these large changes in atmospheric and ocean circulation over the last 2 decades (including also a potentially long-term decline in the Atlantic ocean circulation), have not already disrupted our weather patterns. The map shows this seasons sea surface temperature departures from normal (from NOAA), with a cool East Pacific and unusual patterns over the north Pacific and north Atlantic that are associated with this seasons extreme weather, including drought in California, intense cold in eastern north America and flooding in the UK and Europe. It is unknown at present how long this strengthening of the Pacific trade winds will continue. Constructing projections of the future, the authors find that the current hiatus in global surface temperature will continue as long as the trade winds continue to strengthen. However, following an expected reversal of the trend, the planet can expect rapid warming of the surface to resume in response to the continued rises in greenhouse gas concentrations.
December 2013 Christmas Eve Storm
Strong winds and heavy, driving and continuous rain hit the UK during the pre-Christmas period causing power cuts and travel disruption. The Met Office radar (right) shows the complex series of fronts and troughs including squall lines of intense rainfall and high winds.
Nearly 40mm of rainfall was measured in 18 hours by the automatic rain-gauges at the University of Reading Meteorology field site with a peak wind gust of about 34 m/s (122 km/hour) just after midnight in the early hours of the 24th December, the second and strongest wave of high winds associated with the trailing cold front (see synoptic chart below).
The low pressure area was around 935 mb just to the west of the Scottish Islands, close to a record for the UK (see Eddy Graham's Hebridean blog and a discussion by Christopher Burt on wunderground.com). However, some of the strongest winds battered southern coastal counties of England, associated with the tightly packed isobars ahead of the cold front. It is worth noting that the storm was very well predicted by the Met Office as can be seen from the 72 hour forecast synoptic chart made on the 21st December below.
The storm developed and deepened rapidly as predicted by the Met Office forecast, aided by an extremely strong jet stream, possibly associated with the cold sea surface temperature anomaly in the north Atlantic off the coast of Newfoundland and associated strong gradient in temperature between warm sub tropical regions and cold sub-Arctic regions. The contrast between warm and cold was particularly evident over North America with temperatures at one point reaching 21oC in New York compared to -7oC in Montreal just 500km away (see discussion by Peter Gibbs on BBC Weather). The strength of the jet stream appeared unprecedented at midnight going into the 24th with winds exceeding 250 miles per hour according to Met Desk.
Another storm is predicted to hit the UK on Friday the 27th December but thankfully it looks less severe than the one described above [UPDATE: it was strong enought to blow down my fences!].
As a final thought, a recent comment in Nature Geosciences by Jon Robson and colleagues from the University of Reading may have implications for the North Atlantic jet stream and UK weather (see discussion on Ed Hawkins' Climate Lab Book blog). A slowing of the North Atlantic overturning ocean circulation, long predicted by climate model simulations into the future, may have begun according to a detailed analysis of observations and simulations.
This slow but immense ocean circulation transports vast amounts of heat northwards into the North Atlantic. The predicted slowing of this circulation in response to fresh water input from melting ice and increased precipitation and river flow associated with a warming climate, could plausibly cause an increase in the atmospheric temperatre difference between the warm sub tropics and the cold sub Arctic, with albeit far from certain implications for the strength and position of the jet stream and UK weather patterns. A continued slowdown in the Atlantic Meridional Overturning Circulation could also reduce the number of Atlantic Hurricanes and increase the risk of drought in north Africa.
40 years of Climate Change
I have just completed my 40th year on the planet!
During this short time I became interested in weather following the snowy winter of 1981/82 in Hertfordshire, England, and soon became fascinated by past climate change when ice sheets spread over much of the UK. I learnt about the greenhouse effect in the late 1980s and how human activity was increasing its potency and that pretty much lead me to where I am now, working on understanding variability and change in Earth's climate and teaching what I've learnt and am continuing to learn to students.
During these 40 years, apart from the most obvious changes (specifically that there is less dogs muck on the pavements than there used to be) there have been substantial alterations to our planet that are generally less noticeable.
1) The world population has nearly doubled from just under 4 billion in 1973 to over 7 billion today.
2) A relatively rich minority of this population has disproportionally contributed to a rapid rise in carbon dioxide (CO2) in the atmosphere (see for example slide 43 of Kevin Anderson's talk illustrating Pareto's 80:20 rule). Since 1973 CO2 levels have risen by about 20% from below 330 parts per million by volume (ppm) to nearly 400 ppm (see Mauna Loa record). Atmospheric CO2 concentrations would have risen to much higher levels but for the large proportion of emissions that have been absorbed by the oceans, causing them become less alkaline.
3) The enhanced greenhouse effect from rises in atmospheric CO2 and other greenhouse gases that are also long lived and therefore well mixed in the atmosphere, such as methane and nitrous oxide, have impeded the natural planetary cooling effect through emission of infra-red radiation to space, leading to heating. Around a quarter of this heating has been offset by cooling from aerosol particle pollution which causes more sunlight to be reflected back to space. The overall influence of human activity over the last 40 years has been to cause a steady increase in heating of the planet, estimated in the recent working group I of the 2013 IPCC report as 1.25 Watts per square metre in 1980 (1.25 Joules of energy per second for each of the 510,072,000,000,000 square metres covering the global surface) rising to 2.29 Watts per square metre in 2011.
4) As the planet warms in response to this heating effect (termed a radiative forcing of climate since the radiative energy balance of the planet is being altered) some of this energy is lost to space. The remainder has caused energy to rapidly accumulate, primarily in the oceans. Despite the recent slowdown in warming of the planet's surface over the last 15 years, the oceans have continued to heat up, currently at a rate equivalent to each human on the planet using twenty 2kW kettles to continuously boil the sea.
5) The warming oceans heat up the air above. Global surface air temperatures have risen by about 0.5oC since 1973 (see Met Office observations and the top panel of the graph below). This doesn't sound much but it is actually about 10% of the global temperature change between now and the last glacial periodwhen ice sheets reached as far as Britain (for example simulations of the last glacial maximum 20,000 years ago show global temperatures around 5oC colder than today)
6) Some regions of the planet have warmed more than the global average. Reading and East Berkshire have warmed by nearly 1oC over this period. The Arctic has also warmed substantially and observations indicate around a 40% drop in September Arctic sea ice extent since 1973 - see graph above, middle panel - and ice volume has probably reduced by an even greater extent.
7) Warming oceans have expanded in volume causing sea levels to rise. Melting glaciers over recent decades have added water to the oceans also contributing to the observed increase in sea level over the last 40 years of about 8cm - see graph above, bottom panel.
Given that many of these changes are occurring far away and are averages of many observations it is difficult to notice these changes personally and trust must be placed with the thousands of scientists who have been involved in making and collecting measurements, applying quality controlling and carefully interpreting, evaluating and improving them over time (see for example the Met Office Hadley Centre observations page).
Even more oblique are the projections of future climate change that will arise from our continued emissions of greenhouse gases. As a scientist, presenting a graph showing rising temperature and changing rainfall patterns for the 2080s or 2040s (see IPCC 2013 working group I Summary for Policymakers Figures 7-9) is simple but conceptualising what it will mean to people living in these decades is difficult. What is certain is that when I (hopefully) celebrate my son's 40th birthday in 2045 the world will be a very different place and climate change will also be noticeable over large regions of the globe.
The emergence of climate change from natural year to year fluctuations is likely to affect tropical regions first (see recent research looking at the time of emergence of climate change from natural variability by Camilo Mora and Ed Hawkins for example). The diagram below from the Independent article discussing the Mora research shows what years selected cities are likely to really notice the effects of climate change. As I look forward to the next 40 years, what is certain is that weather and climate will continue to fascinate me, surprise me and, increasingly, concern me.
Another inaccurate Mail on Sunday article on the upcoming IPCC report and climate science
In the run-up to the publication of the upcoming 5th assessment report of the IPCC there is a usual flurry of activity amongst those who have an interest in discrediting the substantial body of evidence pointing to the influence of human activity upon current and future climate change. The Mail on Sunday have published a series of inaccurate articles, including another by David Rose (15th September 2013).
The main claim by the recent Mail on Sunday article is that the rate of global warming since 1951 has been halved since the last IPCC report. This is completely incorrect. As clearly stated in the last IPCC report in 2007, the rate of warming since 1951 was not 0.2oC per decade (as stated by Rose) but 0.13oC per decade.
The 0.2oC per decade observed rate of surface warming applies to 1990-2005 which clearly cannot be compared with the period since 1951 and will not have changed since the last IPCC report. To confuse matters further the Mail tried to infer just this in a "correction" to their article on the 17th September by comparing the 1990-2005 period with 1997-2012, mixing up climate change and natural variability from one decade to the next.
Feeding off the Mail article the Telegraph proclaims "Top climate scientists admit global warming forecasts were wrong" elaborating upon this by saying "...the world is warming at a rate of 0.12C per decade since 1951, compared to a prediction of 0.13C per decade in their last assessment published in 2007." That's just 0.01oC per decade different and makes the title seem completely ridiculous. This is an embarrassment to the serious reporting of climate change elsewhere including writers from these same newspapers including Sam Webb (Mail) and Geoffrey Lean (Telegraph). A weak editorial in the Mail on Sunday further uses the Rose article to reaffirm its apparent agenda against wind farms and in favour of fracking.
Further claims by the Rose article include:
- "large parts of the world were as warm as they are now for decades at a time between 950 and 1250 AD." This may indeed apply to particular regions and seasons but the evidence indicates that warming in the late 20th century was far more coherent and strongly related to rising greenhouse gas concentrations.
- Antarctic sea ice "has actually grown to a new record high" rather than decreasing as predicted by simulations. While this is true when looking at the satellite record since 1979, the increases in Antarctic sea ice are smaller than the rapid decline in Arctic sea ice and are accompanied by a loss in Antarctic land ice. While the changes in Antarctic sea ice are not fully understood, changes in wind patterns relating to natural variability and human factors combined are likely to explain this variability.
- Evidence pointing to an intensification in Hurricanes (or tropical cyclones) has "simply been dropped, without mention" from the latest IPCC report. Considering the improved observations now used, there is certainly not strong evidence that human factors have altered the intensity of tropical cyclones. But climate simulations indicate that by the end of this century the most fierce tropical cyclones will become more intense, in particular the associated rainfall, the increases in which are based upon robust physics included in sophisticated climate simulations.
- The discussion about 90% or 95% confidence that over half of the warming since 1951 is caused by human activities does not seem warranted. While natural fluctuations in the ocean can and do influence decadal rates of surface warming, a large component of the warming since 1951 cannot be explained without including human factors such as burning of fossil fuels.
- The fact that "governments have tabled 1,800 questions and are demanding major revisions" to the IPCC summary for policy makers is overstated and a normal part of the process.
- The Met Office model has an unusually high equilibrium climate sensitivity (the amount of warming far in the future in response to a doubling of CO2 concentrations). While HadGEM2-ES does occupy the upper end of the range in climate model simulations, it cannot be discounted. A low climate sensitivity requires the observed strongly amplifying effects (termed positive feedbacks) of water vapour and surface ice to be counteracted by an equally strong negative feedback. There is no observational evidence for this and it is not yet known whether tropical low altitude clouds will amplify or diminish the warming response to greenhouse gas increases. However, the Met Office model is in fact generally regarded to provide one of the more realistic simulations of these cloud feedbacks and is built upon sound physical processes .
- Rose has suffered "extraordinarily vitriolic attacks from climate commentators". This is not acceptable, nor is abuse sent to climate scientists such as Michael Mann which is often much worse, nor is tax payers money wasted in addressing spurious claims by some parts of the media on the science of climate change.
- The IPCC "are scaling down their estimate" of the lowest range of climate sensitivity. Some new studies have indeed added to our knowledge of how climate will respond to rising greenhouse gases. However, arguing over a few tenths of a oC in climate sensitivity at the bottom of the range masks the real issue which is the expected substantial climate change in response to the continued emissions of greenhouse gases, which are at present following the worst case emissions scenarios.
The climate is highly complex and there will always be debate over what is causing each lump and bump in the temperature record. The recent slowdown in global surface warming is an example of this and the science has responded by moving forward. The latest IPCC report is our best assessment of the science of climate change but certainly cannot be considered perfect. Yet it looks set to build upon the evidence presented in the 2007 report by demonstrating a clear influence of recent warming of climate by human activity which looks set to increase in the near future.
Natural fluctuations in the Pacific ocean explain recent slowdown in global surface warming
A new article published in Nature adds to evidence that the recent slowdown in the rate of global warming at the Earth's surface is explained by natural fluctuations in the ocean and is therefore likely to be a temporary respite from warming in response to rising concentrations of greenhouse gases. This is important since it adds to a great body of research in continuing to confirm the realism of projected dangerous future warming in response human activities such as burning fossil fuels.
In an experiment using a detailed computer simulation of the climate, Kosaka and Xie artificially alter the flow of heat from the tropical east Pacific ocean to the atmosphere above so that sea surface temperatures in this region are forced to agree with observations. This results in the realistic simulation of the recent slowdown in the rate of global surface warming and also reproduces some unusual weather patterns such as the drought that has affected the southern USA.
Although the eastern Pacific is being given the correct ocean temperature in their simulations, this region only covers about 8% of the globe and their experiment highlights the importance of this region in determining natural fluctuations in climate that occur from one decade to the next.
The experiment does not show why the recent hiatus in global surface warming occurs, merely that this type of ocean change can explain the recent slowdown in global surface temperature rise. Free-running climate simulations (used to make future projections of climate change) are able to produce this type of variability but are not designed to predict the exact timings of these changes (see for example illustration by Ed Hawkings).
Rearrangement of heating in the ocean is not dealt with in this study but this work provides more evidence that the recent slowdown in global surface warming is linked to changes in the Pacific ocean, particularly the upper few hundred metres rather than the deep ocean below 1000m (see recent discussion). We are currently investigating how this is happening in the DEEP-C project funded by the Natural Environment Research Council.
Ocean measurements and satellite data show that the planet is continuing to heat up ( Loeb et al. 2012, Nature Geosciences) despite the lack of warming near the Earth's surface. The recent slowdown in global surface warming appears to be explained by natural fluctuations in the Pacific ocean which have caused the additional heating from rising concentrations of greenhouse gases to temporarily affect the layers below the ocean surface rather than surface temperatures of the planet. Over the coming decades, a return to warming of global surface temperatures is extremely likely.
Science Media Centre briefing on the recent slowdown in global surface temperature rise
Global average temperature at the Earth's surface has not risen significantly over the last 10-15 years. Yet based upon observations, the oceans beneath the surface are continuing to warm. While small volcanic eruptions during the 2000s and a minimum in the output of the sun toward the end of that decade have counteracted slightly the heating effects from rising greenhouse gas concentrations (see slide from Piers Forster), a combination of satellite data since 2000 and thousands of automated ocean measurements down to nearly 2000m depth since around 2005 indicate that heat is continuing to accumulate (equivalent to continuous heating by over 150 billion 2 kilo-Watt kettles!). Based on detailed simulations and observations of the ocean surface temperature it is thought that natural changes in ocean circulation have caused heating from rising greenhouse gas concentrations to temporarily warm the deeper ocean (e.g. below 300m) rather than the surface layers which influence Earth's surface temperature (see cartoon below). The precise mechanisms involved are being actively researched (e.g. the NERC DEEP-C project).
ABOVE: A cartoon depicting decades in which heating is strongest in the upper layers of the ocean (e.g. 1980s-90s) and the most recent period in which heating affects deeper layers instead, with little change in surface temperature.
The detailed computer models of our atmosphere, ocean and land that are used to make projections of likely future climate change can represent the fluctuations in ocean circulations that generate decades of stable surface temperatures. They cannot capture the correct timing of these events but they suggest that generally around 2 of such decades can occur every century (see also illustration by Ed Hawkins). Therefore, while it is impossible to predict the timing of these natural fluctuations, the recent slowdown in warming is not out of the ordinary and continued heating of the oceans is consistent with the increasing levels of greenhouse gases in the atmosphere.
It is difficult to say how long the lull in surface warming will continue. One climate simulation with steadily rising CO2 concentrations produces no trend in global surface temperature over a 24 year period. Also, who's to say that an explosive volcano won't erupt over the next few decades (aerosol particles injected into the stratosphere as a result cool the planet by reflecting more sunlight back to space)? However, a return to substantial warming over the next few decades is expected; some decades of rapid warming and others with slow warming will occur as a result of natural fluctuations in the ocean.
I describe some of these aspects in an interview with the Voice of Russia's Tim Ecott. The briefing has also lead to a number of good articles, for example in the Daily Mail and the Independent (see also Telegraph, Guardian, BBC, Times, Financial Times, Herald Scotland and the Associated Press). In addition, the Met Office has developed three detailed reports on the recent pause in surface warming
Comments on a misleading Mail on Sunday article on global warming
A recent story in the Mail on Sunday by David Rose attempts to mislead the public by claiming that "...man-made global warming is a myth...". When all the evidence is considered, this statement is false. As discussed by Real Climate, some aspects of climate are changing more quickly than predicted by climate simulations (e.g. Arctic ice) and others are slower than the projections (e.g. surface temperature over the last 15 years) but observational evidence is consistent with continued man-made global warming.
The article confuses variability and climate change, mixes up processes occurring over millions of years and hundreds of years and ignores the bulk of research on current climate variability and change.
The story does not use a graph from a leaked IPCC report as claimed but instead from the popular and thought-provoking blog by Ed Hawkins, ClimateLabBook, without permission (an acknowledgement has since been placed in the online version of the article) and incorrectly describing some of its details. It shows how rapid warming of climate since the 1970s has slowed over the last 15 years and that warming is below the rate simulated by the vast majority of climate models. This is interesting and is actively being researched, for example in a new National Environment Research Council project, "DEEP-C" which involves the University of Reading, the National Oceanography Centre in Southampton and the Met Office.
Current research, using observations, indicates that heat is continuing to accumulate in the oceans since 2000, consistent with the build-up in greenhouse gases. Global warming has not stopped as the oceans are continuing to heat up. However, in the recent decade, the warming is occurring beneath the ocean surface, suggesting that the current slowing in the rate of surface warming is due to natural ocean variability (although other factors including changes in the solar output, small volcanic eruptions, anthropogenic aerosol emission from Asia and changes in stratospheric water vapour may also have contributed by offsetting some of the radiative forcing from the steadily increasing greenhouse gas concentrations).
An illustration of the effect of variability on global warming appears in another blog by Ed Hawkins. Climate model simulations do produce decades of flat surface temperature in response to anthropogenic global warming. This is down to natural variability (sometimes the build-up of heat is buried further down in the ocean). The graph used in the Mail article averages over lots of models. While some models have higher rates of warming, others display little warming in each decade, because of natural variability; averaging smooths this variability out. This is primarily what gives the spread in surface temperature changes over the last few decades.
The article misrepresented scientists who were consulted (for example, see the comments by Myles Allen) and also contains many other inaccuracies.
For example, the article states: "The evidence shows CO2 levels follow temperature, not the other way around...". This is true over many hundreds to thousands of years, during glacial cycles (changes from glacial ice-age periods to milder interglacial periods like today) which is well known and understood. Atmospheric CO2 and methane changes naturally amplify these glacial cycles. Presently, however, we are generating CO2 from fossil sources, accumulated over millions of years but burnt and emitted back into the atmosphere by human activity over a few hundred years. The build-up of greenhouse gases in the atmosphere over the last hundred years is causing heat to accumulate in the climate system, primarily the oceans. This is why humans are now responsible for global warming of the planet.
Also stated is "...the world can live with these fluctuations in the level of atmospheric carbon...". Since the time in which anatomically modern humans evolved (around 200,000 years ago) CO2 has been below 300 parts per million, until today. CO2 has not been at 4,000 parts per million, as quoted in the article, since 180 million years ago when continents and life on Earth were vastly different to today.
The article also states: "But the scientists behind the theory have a vested interest - its a great way to justify new taxes, get more money and guarantee themselves more work...". Scientists generally choose to study their subject because they are fascinated by it (I have been intrigued by climate change since the age of 10) and not because they want to make lots of money. In the article, the author notes: "I rapidly found myself cast out from the BBC..." and devotes some lines to this. Is this relevant to the article or a personal issue?
The myth that in the 1970s an ice age was predicted is recapitulated in the article. This was not the scientific consensus at the time and primarily reflects media stories (e.g. Time magazine). This and many other myths are examined by Skeptical Science.
A previous article by David Rose appeared in the Mail earlier this year and the expert reaction was similarly damning. Recent blogs and articles, for example by Bad Astronomy, The Carbon Brief, Myles Allen and Skeptical Science, also offer an opinion on the recent Mail story.
Observed changes in climate are in fact consistent with heating of climate due to greenhouse gas emission from human activity which has been offset by additional emissions of pollutant particles called aerosols. The magnitude of future climate change is not certain but the likelihood of damaging climate change appears very likely based upon multiple lines of evidence including basic physics, a multitude of observations and a vast array of detailed physics-based global simulations and experiments.
2012 wettest year on record in England
The Met Office confirmed that in 2012 England had its wettest year since records began in 1910 and the UK its 2nd wettest behin 2000 (see Met Office data for the UK and statistics for Reading). This was despite a dry first 3 months of 2012 (particularly March which ended 2 years of below average reainfall) and a relatively dry year for NW Scotland (see figure from the Met Office below).
It is interesting to note that 4 of the 5 wettest years on record occurred since 2000 (while 2010 and in particular 2003 were unusually dry). December, April and (in particular) June were exceptionally wet with associated flooding associated with intense summer downpours and prolonged rainfall in the summer, early Autumn and December, resulting in flooding and disruption to transport.
Why was it so wet?
The jet stream, a ribbon of fast-moving (>160 km/hour) winds around 10km above our heads, is responsible for directing weather systems over the UK or deflecting them away from the UK. It meanders north and south over thousands of km, causing above average rain in some regions and below average in others, depending upon whether it is further north or further south than usual. For example, while the UK was enduring persistent rainfall, the north America was suffering from summer heat-waves and drought. A dramatic shift in the average position of the jet stream from March to April 2012 can broadly explain why conditions changed from that of drought to flooding in the UK.
What caused the jet stream to change and can we expect more years like 2012 in the future?
The jet stream owes its existence to a rapid decline in temperature from the warm sub-tropics to the polar regions. This temperature gradient, combined with the spin of the Earth, produce these upper level jets. Therefore, changes in this temperature gradient can certainly affect where the jet stream is and how "saggy" the loops and meanders are. Much of the variability is a result of natural fluctuations: we will always get very wet years and very dry years. This year, there were unusually warm waters in the north Atlantic, particularly off the coast of Newfoundland (see figure below).
Changes in north Atlantic sea surface temperature oscillate from warm to cold over many decades and this is thought to influence weather over the UK, in particular for summer. However, an additional warming trend, relating to the build up of greenhouse gases in the atmosphere, is also having an impact on our weather, and will increasingly do so in the future as global temperatures rise.
Flooding in the UK is often associated with large transports of moisture from the warm sub-tropics, referred to recently as Atmospheric Rivers. For example, microwave satellite imagery processed by REMSS shows strong winds combined with high water vapour amounts directed towards the UK on 21 December (see figure below), resulting in heavy, prolonged rainfall and further flooding.
As air temperatures increase, so does atmospheric gaseous water vapour, at the rate of around 7% for each oC of warming. This moisture is the fuel for storms which suck in this invisible water and following condensation of water doplets and formation of ice crystals, produce heavy rainfall. Therefore as moisture levels increase, so does the potential for more intense rainfall. This suggest that in the future, while we will always have wet and dry years associated with shifts in the jet stream, when it is wet, the rainfall will arrive in heavier bursts, with more flooding possible.
There are many other possible influences on our weather patters, some of which are natural, like changes in the sun or oscillations in the ocean, and some of which are strongly influenced by humans such as greenhouse gas caused increases in temperature and moisture and the melting of Arctic ice with its possible influence on ocean temperature patterns and the jet stream. However, computer simulations of the future combined with basic physics tells us that we can expect more flooding and more drought globally as a result of man-made climate warming.
More Flooding in the UK, September 24/25
More flooding strikes the UK as an intense low pressure system (see Met Office Synoptic Chart), with lots of associated atmospheric moisture, tracks slowly over the UK from the south. Here is a Met Office radar animation of the rainfall intensity (pink is heavy, red is very heavy) from Sunday 23rd to Monday 24th September:
The depression is not the remnants Hurricane Nadine which is centred near the Azores (as noted by the Met Office). However, Remote Sensing Systems satellite passive microwave water vapour imagary indicates that large amounts of moisture (the fuel for heavy rainfall) were being steered up towards the UK with certainly an influence from the decaying Huricane.
A vertical cross section of the system on 23 September over the Bay of Biscay from active CloudSAT radar data shows the remains of Nadine centre left and convection in the warm sector to the right over northern Spain.
Flood warnings were issued over many parts of the UK (see BBC report and update). While recent research at the University of Reading has linked flooding in the UK with intense flows of atmospheric moisture during the winter half of the year (October-March), the large quantities of water vapour transported by the atmosphere are also key in detemining the nature of summer flooding events.
The global water cycle describes the movement of water in its gaseous, liquid and frozen states around the climate system: for example, evaporation over the oceans, transport of the gaseous vapour by the atmosphere, condensation as cloud with precipitation returning fresh water to the surface (rainfall, snowfall, etc) and run-off through rivers back into the oceans (see Figure below). Societies and ecosystems are highly sensitive to the availability of fresh water. Too much at once can cause damage (e.g. flooding) while too little over sustained periods of time can deplete water reserves. Damage from both have been experienced over the last year in the UK and globally: recent occurrence of extreme weather understandably heightens concern on the human influence on and vulnerability to such events.Illustration from Trenberth et al., 2007, Estimates of the global water budget and its annual cycle using observational and model data, Journal of Hydrometeorology.
One of the most concerning aspects of climate change are the changes in the water cycle that accompany sustained global warming or cooling. Based on relatively simple physics, we know that there will be more rainfall globally as temperatures warm. We also know that, on average, increased rainfall will affect the already wet equatorial regions and the higher latitude regions. Both of these regimes borrow water from further afield, while the regions that provide that water, through evaporation, for example over the dry sub-tropics, will on average experience drying trends. The latest state-of-the-art climate simulations from the 5th phase of the Coupled Model Inter-comparison Project (CMIP5) demonstrate these projected patterns which are explained quite well by simple physics.
To really build confidence in our predictions of changes in the global water cycle, the realism of simulations of climate and its variability must be established using available observations of rainfall. In a paper published in Geophysical Research Letters (lead by Chunlei Liu at the University of Reading and in collaboration with NASA/Goddard Space Flight Center) we looked in detail at how well the new CMIP5 simulations of precipitation compare with observations taken from satellite measurements and rain-gauges at the land surface. We tested two types of experiments: ones which simulate ocean variability as well as changes in the atmosphere over the last 100 or so years (the standard CMIP "historical" experiments) and another set in which the observed changes in sea surface temperature and sea ice distribution are fed in as input to atmosphere-only simulations over the period 1979-2008 (called AMIP). Both experiments are given realistic changes in climate forcing agents (e.g. carbon dioxide and reflective volcanic or human produced sulphate aerosols). The AMIP experiments are the most consistent with the observations since the natural warming and cooling cycles match the observational record. However, they don't contain ocean "feedbacks" which influence the progression of climate that are required to simulate future climate change; this is why it is important to look also at the CMIP experiments.
Our main findings were as follows:
1) Bias in the simulations: Both CMIP and AMIP simulations appear to overestimate rainfall over the tropical oceans, by between 19% and 40% depending upon which agencies simulator was used (for example, NASA Goddard Institute for Space Studies GISS-ER simulations gave the largest amount of rainfall while the French IPSL CM5A-LR were closest to the observed estimates). The biases were the largest over the wet regions of the tropical west Pacific and Indian Oceans.
2) Global Precipitation Rises with Warming: When considering the natural warming and cooling cycles, relating to the El Niño Southern Oscillation (ENSO), AMIP5 simulated increasing global precipitation of 1.6-3.6 % per oC increase in global temperature depending on which agencies simulation was considered. The CMIP5 simulations were similar but slightly lower while the observations from the Global Precipitation Climatology Project (GPCP) were at the upper end of this range.
3) The "Rich" get "Richer": The increase in precipitation with natural warming cycles originated primarily from the wet tropical ocean regions: in these regions the observed response was around 3 times larger than the simulated relationship. Over the the remainder of the tropical oceans, where less rain falls on average, both observations and simulations show a decrease in rainfall when tropical temperatures are warmer.
4) Rainfall Moves between Ocean and Land: When there is more rainfall over the ocean (during El Niño) there is less rainfall over land.
Finally, the result that I found most interesting is shown in the Figure below:
5) The AMIP simulations accurately capture the observed changes in rainfall over tropical land regions: there is up to 10% more rainfall than usual in La Niña years and up to 10% less rainfall in El Niño years. This is despite the AMIP simulations predicting their own land surface temperatures (only the ocean temperatures are provided as input). This suggests that the simulations are realistically capturing the processes determining changes in rainfall over land.
6) A discrepancy exists between the satellite observations and the simulations over the oceans: before about 1996 there is almost no agreement in the year-to-year fluctuations between the observations and simulations over the tropical oceans. After 1996 there is much better agreement but the observations show larger variation.
We discuss in the paper some possible reasons for this. One is that observing rainfall over the oceans is a challenge and there may be some inadequacies in the observations (although it is not clear why things improve after 1995). Another explanation could be that there has been a change in the nature of the natural ENSO cycle which, combined with the large simulation biases in the west Pacific and Indian ocean, may conspire to generate alternate periods of good and poor agreement.
The work is part of a larger project (PAGODA) looking at global changes in the water cycle which itself is part of a Natural Environment Research Council (NERC) program to study the Changing Water Cycle. The work also contributed to the NERC National Centre for Atmospheric Sciences (NCAS) and National Centre for Earth Observations (NCEO). The plan for further work is to look into and understand these discrepancies in more detail. Combining the observations and simulations is an effective way of improving understanding of our climate system in addition to assessing the fidelity of the observing systems and simulations, crucial for predicting future changes in the global water cycle.
Flooding in Newcastle, June 28 2012
A series of intense thunderstorms crossed England during 28th June 2012 associated with a series of cold fronts (see synoptic chart below). In Reading it was mostly dry, hot and sultry (up to 26.5oC just after 4pm and over 90% humidty in the early hours at the Reading Meteorology field site).
The radar images (right) show some extremely intense rainfall rates over the Midlands (large hailstones were reported) while intense rainfall over Northumberland caused serious flooding in Newcastle (where the water failed to drain away rapidly enough) and a landslide closed the East coast trainline between Newcastle and Edinburgh causing difficulties for the organisors of a Changing Water Cycle workshop in Reading (which covered, amongst other things, extreme rainfall) who had to return to Edinburgh!
Data from the European Centre for Medium Range Weather Forecasts "Interim" reanalysis of the atmosphere 2 days before the flooding events show a toungue of very moist air extending from the sub tropics towards the UK (left). Recent research at the University of Reading has linked flooding in the UK with Atmospheric Rivers of moisture, like the ones shown, but for the winter half of the year (October-March). The diagram also indicates that these atmospheric flows are also associated with summer flooding events.
Although the flooding and intense rainfall was caused by the weather pattern (which wasn't unusual for summer in the UK) research (presented at the workshop) shows that a warmer atmosphere contains greater quantities of invisible water vapour (see animation), the fuel for intense rainfall, and this is why warming due to emissions of greenhouse gases is very likely to result in heavy rainfall becoming even heavier in the future
Flooding in Southern England, June 2012
Here is an animation of the Met Office 'classic' radar images from 10-12 June 2012 (right) which shows an arc of intense rainfall associated with a low pressure system almost stationary over South East England. Following severe localised flooding in mid Wales a few days earlier, further flooding occurred in urban areas along the Southern coast, in particular West Sussex.
The flooding has been linked to an unusually southern track of the jet stream: specifically a deepening, stationary low pressure system over the Southern UK with rising air, cloud formation and heavy rainfall lingering over particular locations. The larger totals of atmospheric water vapour in the relatively warm atmosphere fuels the intense rainfall.
In Reading, the Meteorology department field site automated gauge recorded 25mm in 11 hours on the 11th (below) before the rainfall totals went off the scale (perhaps it thought it needed a break). The standard rain gauge at the Meteorology Department Atmospheric Observatory measured 40.7mm of rain over the 10-11th June period (observations taken at 9am). At the time of writing, more heavy rain was expected on the Friday, just in time for the Meteorology department BBQ and Barn Dance!
In an article published in Nature Geosciences we present an analysis of the heat entering our planet since 2000, measured at the top of the atmosphere by satellite instruments and down to a depth of 1800m within the world's oceans by automated profiling floats.
Despite an apparent break in the warming trend of the global ocean surface since the warm El Niño year of 1998 (see Figure, right), our analysis (lead by Dr Norman Loeb of the NASA Langley research centre) suggests that heat has continuing to build up at the rate of 0.5 Watts for each square metre of the Earth's surface - that is equivalent to the total heat produced by over 250 billion kiloWatt electric heaters distributed over the globe (assuming a global area of 510 million km2). This is important since it suggests that while surface warming appears to be absent over the last decade, energy continued to build up below the ocean surface, at a rate that is consistent with the effects of increased greenhouse gas concentrations in the atmosphere.
Global climate change results from an imbalance between the amount of sunlight absorbed by Earth and the thermal radiation emitted back to space. Elevated concentrations of greenhouse gases, due primarily to the burning of fossil fuels, have reduced the efficiency at which Earth can cool to space through thermal radiative emission, resulting in a positive energy imbalance. Once this excess energy enters Earth's climate, small amounts are used heating up the atmosphere and the land as well as melting ice, but the bulk enters the oceans which have the capacity to store vast quantities of energy. Therefore fluctuations in radiative energy entering at the top of Earth's atmosphere (see previous post) must vary in unison with changes in ocean heating rate.
The effects of El Niño and La Niña - the climatic phenomena affecting normal weather patterns across the tropical Pacific - are enough to disturb this steady build up of heat. Yet scientists had previously puzzled over additional "missing energy" that was detected entering the planet but which appeared not to arrive in the oceans. Our analysis suggest that changes to the way sea temperatures are measured, improvements in the satellite data products and substantial statistical margins of error are enough to account for this discrepancy. Furthermore, they confirm that energy has indeed been accumulating in Earth's climate since 2000 and that much of this "excess energy" has been continuing to heat the sub-surface ocean.
Work by David Lavers and co-authors has linked flooding in UK river catchements with distinct flows of warm, moist air termed moisture conveyors or Atmospheric Rivers (see New Scientist article).
These moisture conveyors carry large quantities of atmospheric moisture, the vast majority of which is invisible vapour, and this provides the fuel for intense precipitation. When moisture laden air masses (see also global animation of water vapour from the ERA interim reanalysis during 2011) are raised over mountains, the air cools and since cooler air carries less water vapour, rapid condensation of droplets can initiate heavy rainfall.
Over a period of a day or so, these heavy rainfall events can rain out much more water than is contained in each vertical column of air. This is only possible with the continual supply of moisture from further afield. In this work we have identified these warm, moisture conveyors or atmospheric rivers of moisture that supply large quantities of water vapour over a period of a few days. In the case shown in the Figure, this particular event caused substantial flooding in Cumbria (see also animation of atmospheric water totals from ERA Interm reanalysis).
Flooding will be particularly acute in catchments with rapid rainfall-river flow response times such as those found in the north and west of Britain. More results are described in the paper in Geophysical Research Letters
Since water vapour is almost certain to increase on average in a warming world, there are the implications that these events may become more severe and this is research that we are interested in conducting in the future. This research was conducted as part of the HydEF project funded through the Natural Environment Research Council's Changing Water Cycle program (read more on the departments Weather and Climate Discussion Blog).
University of Reading joins International Space Innovation Centre
Complex mathematical representations of Earth's atmosphere, oceans and land are required to make realistic predictions of future weather and climate. Satellite data is becoming increasingly important in evaluating and improving the physical representation of simulations of planet Earth that are made by these detailed computer models. For example, since 2003, Met Office weather forecast model simulations have been routinely scrutinised through comparisons with GERB satellite measurements of the Earth's radiative energy balance. Detailed analysis has been undertaken to understand and improve the simulation of cloud processes, examining the radiative properties of cirrus cloud from aircraft condensation trails and identifying the the greenhouse effect of desert dust.
As part of the commitment to the exploitation of satellite data in monitoring and measuring the workings of planet Earth's environment, the University of Reading has joined ISIC. Read more...Above: Radiative energy (units: Wm-2) emanating from planet Earth at 6am GMT on Monday 24 October 2011 as simulated by the Met Office global forecast model and the Geostationary Earth Radiation Budget (GERB) satellite instrument.
Dark regions in the image denote high altitude cloud, with cold tops that only weakly emit thermal infra-red (or longwave) radiative energy out to space. Relatively cloud-free, hot regions, such as Saudi Arabia in the image, emit strongly in the longwave part of the electromagnetic spectrum and appear light in the image above. This image was produced as part of the joint University of Reading and Met Office SINERGEE project (see article describing method), funded by the Natural Environment Research Council.
Intensified flows of moisture into the tropical rainy belt
Moisture is the fuel for tropical storms. Since they typically rain out in a day more water than is contained in the column of atmosphere, rain storms rely on a convergence of water vapour from surrounding regions. As part of the Natural Environment Research Council PREPARE project, work lead by project scientist, Matthias Zahn, has indicated an intensification of the inflow of moisture at low levels and outflow higher up in the tropical atmosphere over the last 2 decades (see Figure which depicts changes in moisture inflow into the tropical wet regions 1989-2008).
We found that detailed calculations every 6 hours were required to accurately follow the flows of moisture and our results have implications for the tropical water cycle and the intensity of rainfall. Further work is required to understand the changes in tropical circulation and its implications for changes in precipitation patterns. This work was published in the Journal of Geophysical Research.
Changes in Earth's radiative energy balance 1985-2010
In an article published in Meteorological Applications I put together the latest estimates of changes in the amount of energy entering the top of Earth's atmosphere. This comprises incoming sunlight, the outgoing reflected sunlight and the outgoing thermal emission of longwave radiation. The diagram below shows how this net flux changes from month to month (after removing the normal seasonal changes) based on satellite data (ERBS and CERES) and from reanalysis data which combines weather forecast models with observations:Estimates of near-global changes in (a) the net flux of energy into the top of the atmosphere (Net) and (b) the cloud radiative effect (NetCF) on this energy balance calculated by removing the simulated clear-sky fluxes (both in Watts per metre squared) since 1985 (the region considered is 60oS-60oN).
The warm El Niño years of 1998 and 2010 are characterised by negative departures from the norm. This is a result of natural shifts in the distributions of water vapour, cloud and surface temperature. Also prominent is a drop in net flux following the Pinatubo volcanic eruption in 1991 visible in the ERBS satellite data. Reflective particles called aerosol entering the stratosphere following the eruption increased the reflectivity of the planet (this is not seen in the reanalysis model since the volcano was not included and so the NetCF shows the influence of aerosol and cloud on the radiation balance; climate models can actually capture volcanic induced changes in the energy budget quite well).
Future work is endeavouring to understanding in more detail the bumps and dips in this graph which may tell us more about how the climate responds to small yet persistent radiative imbalances that determine whether our climate warms of cools. At present, human influences on the atmosphere have caused the Earth to receive more energy each year than it loses to space, resulting in a heating of the oceans.
Richard P. Allan Location: Department of Meteorology (2U15)