Physical science of climate change

Key points of relevance to society from the physical science of climate change

This blog post is based on the headline statements of the recent report (WG1 Sixth Assessment Report) of the Intergovermental Panel on Climate Change.


The world's climates have changed over the last 100 years. The world is probably warmer now than it has been for millennia. It is very likely that most of the observed warming since the mid-20th century has been due to increasing greenhouse gas concentrations in the atmosphere, particularly carbon dioxide from combustion of fossil fuels.

© Jonathan Gregory 2003

© Ed Hawkins, based on IPCC 2021 (WG1 Sixth Assessment Report)

Glaciers worldwide have been contracting since the 19th century. The moraine along the right-hand side of this glacier, which is the Glacier de Moiry in the Valais, Switzerland, shows its size at that time. This is a clear indication of widespread warming, and makes a substantial contribution to global mean sea-level rise. The blue line shows a global surface temperature reconstruction from various indirect kinds of evidence ("proxies", such as tree rings). The shading shows the extent to which various datasets agree. The black line shows the instrumental record (measurements with thermometers). The red line shows how the atmospheric carbon dioxide concentration has risen since the Industrial Revolution.

The underlying physical reason for this is well-understood: it is because carbon dioxide and other greenhouse gases absorb heat radiation; thus, raising their concentration in the atmosphere increases the Earth's thermal insulation from space. It is analogous to the effect of cavity wall insulation: a well-insulated house will have a higher temperature indoors than a poorly insulated one, if the same amount of heat is used. In fact, when we quantify the amount by which the mean temperature at the surface of the Earth is raised by increasing the heating due to greenhouse gases, we use a number whose units (W m⁻² °C⁻¹) are the same as the units of the "u-value" which is used to measure the thermal insulation of walls and windows of a house.

The most important application of climate science is to predict changes in climate and sea level in coming decades and centuries. To do this reliably and precisely, we need an improved quantitative understanding of the processes at work.

The magnitude of predicted global climate change

It is predicted that global mean temperature will rise considerably more during the 21st century than it did in the 20th, principally because of the increasing rate of emission of carbon dioxide. Because we don't know what the future emissions will be, predictions are made assuming various emissions scenarios. Unsurprisingly, climate change is predicted to be larger under scenarios of greater emissions.

The main tool for making such predictions are computer programs, usually referred to as "climate models" (and more specifically as "atmosphere-ocean general circulation models", AOGCMs), which encode mathematical descriptions of our physical understanding of the behaviour of the climate system, based on observations and theory. These are complex programs, hundreds of thousands of lines long, because there is a lot we understand. On the other hand, there is also a lot that we don't understand. Models which make different assumptions about things which aren't well-known make consequently different predictions. Most of my research is concerned with aspects of understanding the causes of these model differences and which predictions are most realistic, by comparison of the models with each other and with climate change which has been observed in the last century and which has naturally occurred in the earlier history of the Earth.

© IPCC 2021 (WG1 Sixth Assessment Report) © IPCC 2013 (WG1 Fifth Assessment Report)

Global mean temperature change (left) and global mean sea level rise (right) predicted during the 21st century under several different emissions scenarios.

Different models give a range of predictions of climate change for any given emissions scenario. Much of the the spread of model predictions can be attributed to differences among models in two categories of phenomena. Climate feedbacks are changes occurring in the atmosphere or at the surface which tend to reinforce or mitigate climate change. For example, the reduction in the area of snow which accompanies warming of the climate gives a positive feedback, because it reduces the reflection of sunlight, leading to more heating of the climate. Heat uptake by the ocean strongly influences the rate of climate change. The ocean has a large capacity to absorb heat, and at present it is retarding global temperature rise.

Warming of the ocean leads also to sea level rise due to thermal expansion of the sea water. As glaciers worldwide and the ice-sheets of Greenland and Antarctica continue to contract, the ice which they lose is converted to water in the ocean, and thus also contributes to sea level rise.

Geographical distribution of predicted changes in climate and sea level

Model predictions for the 21st century indicate that climate and sea level change will not be geographically uniform. The variations are large compared with the global average change. For practical purposes, it is regional change which matters and it is therefore essential to understand the mechanisms which determine the geographical patterns.

Climate models agree qualitatively with regard to major features, although not in detail. Projected warming over the land is on average greater than over the sea, and warming is enhanced at high northern latitudes. Precipitation is projected to increase over high latitudes and parts of the monsoon regions, but decrease over parts of the subtropics. Projected changes are larger under higher emissions scenarios.

© IPCC 2021 (WG1 Sixth Assessment Report)

Projections of annual-mean surface temperature change (left) and precipitation change (right) with respect to pre-industrial for global warming of 4°C.

Jonathan Gregory