The Earth's atmosphere has a natural greenhouse effect, without which the global mean surface temperature would be about 33 degrees C lower and life would not be possible. Human activities have increased atmospheric concentrations of carbon dioxide, methane, and other gases in trace amounts. This has enhanced the greenhouse effect, resulting in surface warming. Were it not for the partly offsetting effects of increased aerosol concentrations, the increase in global mean surface temperature over the past 100 years would be larger than observed. Continued surface warming through the 21st century is inevitable and will likely have widespread ecological impacts. The magnitude and rate of warming for the global average will be largely dictated by the strength and direction of climate feedbacks, thermal inertia of the oceans, the rate of greenhouse gas emissions, and aerosol concentrations. Because of regional expressions of climate feedbacks, changes in atmospheric circulation, and a suite of other factors, the magnitude and rate of warming and changes in other key climate elements, such as precipitation, will not be uniform across the planet. For example, due to loss of its floating sea-ice cover, the Arctic will warm the most.
This article provides a brief overview over some of the main findings in the most recent IPCC WG I report and in articles published after the report. It is argued that the conclusions in the report on observed climate variations and trends during the last 100 years have been largely confirmed or even reinforced by the most recent studies. Concerning future climate change, new analyses of possible changes in sea-level, which take melting land ice into account, indicate that the global sea level may rise as much as one meter within the present century.
Humanity has always lived under the threat of disasters such as famine. Now that these threats have diminished considerably in the West, it seems like people need a new scare that can be shared, thereby having a uniting effect. The possible impact of an increased atmospheric greenhouse gas concentration seems to have taken over this role. However, new dating techniques and numerous new studies have now added information that can bring about a reevaluation of the opinion that it is only human activity that can explain recent climatic changes. A distinction between trends and variability in climate is only possible if long-term records can be studied. Greenland ice core data yield well-dated information about climate over an extended period that, seen together with other data series, indicates that large, probably global scale changes have occurred at numerous times in the past. The warming during the past 100 y is not likely to be unique.