Understanding climate change

The world's climates have changed over the last 100 years, and it is likely that most of the observed warming over the last 50 years has been due to increasing greenhouse gas concentrations, particularly carbon dioxide from fossil fuels. The northern hemisphere is probably warmer now than at any time in the last millennium. We seek to understand and quantify the processes which influence climate and sea level change over decades and longer timescales, by analysis of the results of computer climate models and comparison with climate change that has naturally occurred in the past history of the Earth. Better understanding will help reduce the uncertainty in predictions for coming centuries.

© Jonathan Gregory 2002
© IPCC 2001
Figure 2.20 of IPCC WG1 TAR
Glaciers worldwide have been contracting since the 19th century. (The moraine along the right-hand side of this glacier shows its size at that time.) This is a clear indication of widespread warming, and makes a substantial contribution to global sea-level rise. Millennial Northern Hemisphere temperature reconstruction (blue) and instrumental data (red) from AD 1000 to 1999, adapted from Mann et al. (1999). Smoother version of blue series (black), linear trend from AD 1000 to 1850 (purple-dashed) and two standard error limits (grey shaded) are shown.

Geographical distribution of change 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. For temperature change, climate models agree qualitatively with regard to some features, for instance that warming over the land is on average greater than over the sea, and that warming is enhanced at high northern latitudes. However, the models do not agree quantitatively. For sea level change, there is essentially no similarity between the patterns predicted by various models. To address these disagreements, we need to discover why the model predictions differ as a first step to establishing which are most realistic.

© Crown Copyright 2003, courtesy of the Hadley Centre The maps show the changes in surface air temperature (left, in degrees Celsius) and sea level (right, with respect to an arbitrary baseline) predicted for the end of the 21st century using the Hadley Centre climate model HadCM3 with the A1FI scenario for greenhouse gas and other emissions.

Ocean heat uptake

Over decades and centuries, the ocean strongly influences the rate of climate change, because it has a large capacity to absorb heat. At present, the ocean is retarding global temperature rise, since it warms up slowly, but this has the corollary that the climate will continue to change for a long time after greenhouse gas concentrations are stabilised in the atmosphere. Warming of the ocean leads also to sea level rise due to thermal expansion of the sea water. As with surface climate change, there are substantial differences among state-of-the-art models in ocean heat uptake. Analysis of global model results will benefit from comparison with oceanographic theory and observations of recent changes in the ocean interior.

© Crown Copyright 2003, courtesy of the Hadley Centre. Global average ocean temperature change (degrees Celsius) in a HadCM3 experiment following scenario IS92a for future greenhouse gas increases. The warming spreads downwards from the surface. The rate of warming is larger in the 21st than the 20th century.	A comparison of climate response parameter (red) and ocean heat uptake efficiency (green) in various climate models, adapted from Raper et al. (2002).

Atlantic overturning circulation

The meridional overturning circulation in the Atlantic (associated with the surface current often referred to as the Gulf Stream) transports heat northwards from the tropics and helps keep Europe warm. A weakening of the circulation strength could therefore cool our climate. The overturning is driven by north-south density contrast and most global models suggest that it will weaken under greenhouse scenarios, because of a combination of influences tending to reduce the density of North Atlantic waters: reduced heat loss, increased precipitation and river inflow, increased melting of the Greenland ice sheet. The relative importance of these terms in different models and the sensitivity of the circulation to the consequent temperature and salinity changes are the subjects of ongoing collaborative projects with international and UK partners.

Jonathan Gregory

© Jonathan Gregory 2002	© IPCC 2001 Figure 2.20 of IPCC WG1 TAR
Glaciers worldwide have been contracting since the 19th century. (The moraine along the right-hand side of this glacier shows its size at that time.) This is a clear indication of widespread warming, and makes a substantial contribution to global sea-level rise.	Millennial Northern Hemisphere temperature reconstruction (blue) and instrumental data (red) from AD 1000 to 1999, adapted from Mann et al. (1999). Smoother version of blue series (black), linear trend from AD 1000 to 1850 (purple-dashed) and two standard error limits (grey shaded) are shown.