Why study past climate variability?
To assess the role of global warming associated with the twentieth century temperature trend (see below), it is of particular importance to identify and quantify the contribution of anthropogenic influences. Therefore, detailed insight into high-resolution temporal and spatial patterns of natural climate variability prior to the industrial era is essential for assessing late twentieth century climate change. Since instrumental data only are available for a relatively short period, generally within the period when man has influenced climate, the use of climate proxy records is a necessity for detection and understanding of past climate variability. High-resolution reconstructions of past climate are mainly based on corals, tree rings, sediments, and ice cores. These reconstructions are calibrated against observed records, but the reliability of a reconstruction may also be inferred from comparisons with modelling results from runs of coupled atmosphere-ocean circulation models. Analyses of long proxy records can emphasize distinctions between natural long-term modes and anthropogenic induced climate variability.
Overview of global warming (from Wikipedia, the free encyclopedia)
The scientific consensus on global warming is that the Earth is warming, and that humanity's greenhouse gas emissions are making a significant contribution. This consensus is summarized by the findings of the Intergovernmental Panel on Climate Change (IPCC). In the Third Assessment Report, the IPCC concluded that "most of the warming observed over the last 50 years is attributable to human activities". The global temperature on both land and sea has increased by 0.6 ± 0.2 °C over the past century. At the same time, the volume of atmospheric carbon dioxide has increased from around 280 parts per million in 1800 to around 315 in 1958, 367 in 2000 (a 31% increase over 200 years), and about 380 in 2006. Other greenhouse gas emissions have also increased. Future carbon dioxide levels are expected to continue rising due to ongoing fossil fuel usage, though the actual trajectory will depend on uncertain economic, sociological, technological, and natural developments.
Climate models, driven by estimates of increasing carbon dioxide and to a lesser extent by generally decreasing sulphate aerosols, predict temperatures will increase by between 1.4 and 5.8 °C in the period 1990 to 2100. Much of this uncertainty results from not knowing future carbon dioxide emissions, but there is also uncertainty about the accuracy of climate models. Climate commitment studies predict that, even if levels of greenhouse gases and solar activity were to remain constant, the global climate is committed to 0.5 °C of warming — some model results are as high as 1.0 °C — over the next one hundred years due to the lag in warming caused by the oceans. Note that although most studies focus on the period up to 2100, warming would be expected to continue past then, since CO2 has a long average atmospheric lifetime.