IPCC - The Third Assessment Report

Working Group I is concerned with the science behind climate change. It is charged with reviewing evidence of past climate change, signs of human-caused climate change, and modelling future climate change.

Key Findings

Climate Change is Happening

The world is warming and the climate system is changing.

As shown in the first part of this figure, the global average surface temperature increased over the 20th century by 0.6ºC, after processing the data to remove various biases including urban heat island effects.

The second part of the figure shows a compilation of northern hemisphere temperatures over the past 1000 years. Clearly, average global temperatures are increasing beyond the range observed for the past millennium.

Warming has already had effects

Snow extent has decreased by about 10% since the 1960's, while mountain glaciers have retreated rapidly and Northern Hemisphere sea ice cover has substantially declined in recent decades.

Global average sea level rose by 10 to 20cm during the 20th century, and the amount of heat stored in the ocean has measurably increased since observations began in the 1950's.

Rainfall patterns have changed in the Northern Hemisphere, with generally more rain at high latitudes and near the equator and less in the sub-tropics. There has also been a shift to heavier rainfalls at high latitudes.

Warm El Niño episodes have been more frequent, persistent and intense since the mid-1970's than during the previous 100 years.

Greenhouse gas emissions and their atmospheric effects are increasing

The atmospheric concentration of CO2 is now 31% higher than it was in 1750, probably the highest it has been for the past 20 million years, and is accelerating. About three-quarters of the increase is from fossil fuel burning, while the rest is mostly due to deforestation. Atmospheric methane has increased even more dramatically, by 151% since 1750. Nitrous oxide and synthetic greenhouse gases (halocarbons) also continue to rise.

Human activity is responsible

Human activities are causing climate change. One of the most important features of the Third Assessment Report is the strengthening of the conclusion that human activity is driving the observed climate change.

Climate models have improved greatly since the Second Assessment Report (1996), and now include factors such as water vapour, ocean heat transport and sea-ice dynamics.

As shown in these three figures, models which take into account only natural climate factors during the last century fail to match the observed increase in global temperatures, while modeling the effect of fossil fuel burning predicts too much climate change. However, models which incorporate both human and natural factors match the observed record very well. This data shows that human influence is now the main driving force in climate change, with natural rhythms providing a moderating effect.

Humans will continue to cause climate change through the 21st century

According to climate models, atmospheric CO2 in 2100 will be between 540ppm, if we begin to phase out fossil fuel use immeditely, and 970ppm, if there is no effort to shift away from fossil fuels. All indications are that the average global temperature and sea level will continue to rise.

Temperatures are predicted to rise by 1.4 to 5.8C by 2100, a much more rapid rate of change than that observed during the 20th century. Most land areas will warm more rapidy than the global average, especially at high latitudes in the Northern Hemisphere. Extreme events are likely to increase, and droughts and floods will become more common in many regions. Many alpine glaciers will disappear, snow cover and sea ice extent will continue to wither, and sea level is projected to rise by 0.09 to 0.88m.

The effects of our emissions will be felt far into the future. Most greenhouse gases (CO2, N2O, halocarbons) can persist in the atmosphere for several centuries and will continue to change climate until they are stabilized. Global sea level will contine to rise for hundreds of years, as the oceans adjust to warmer temperatures. Continental ice sheets will also continue to react over centuries - for instance, it is believed that the Greenland Ice sheet could melt completely, raising global sea levels by 3m.

As the scientific underpinning of climate change research advances, so does our understanding of the impacts the future holds in store.

Working Group II: Impacts, Adaptation, and Vulnerability