Paleoclimate records of the last 1000 years indicate that climate varies naturally due to factors such as solar and volcanic activity. These factors are called "forcings" because they drive or "force" the climate system to change. Could these forcings, rather than the human-caused rise in greenhouse gases, have caused Global Warming since the 19th century?
Scientists use various techniques to study this question. One method to study past, present and future effects of these forcings - solar variations, explosive volcanic activity, greenhouse gas concentrations and aerosol particles - is to use models of the full climate system.
What are climate models?Climate models are computer programs that apply physical laws to calculate how climate has changed in the past and may change in the future. Models range from relatively simple ones, which represent only the most essential processes at a coarse spatial resolution, to complex ones, which include many additional important interactions between the atmosphere, ocean, sea ice and land surface operating at regional scales. These models require as input information about forcings such as solar variations, volcanic activity and greenhouse gas concentrations, usually in the form of time series. They compute temperature, precipitation, and other climate variables.
One important test of climate models is to simulate past climate variability and change. When model results and paleoclimate records agree, this provides confidence that the models are reasonable representations of the real climate, and that the dominant forcings that drive climate variability and change have been identified.
Summary of ResultsSimulations of the last 1000 years have been completed with several different models. Although some of the details are different, they all show several similar trends in Northern Hemisphere climate: relative warmth before the 14th century followed by cold periods between the 15th and early 19th centuries. The warming of the 20th century is, given the perspective of the previous millennium, unprecedented. This general picture agrees with the independent proxy reconstructions. Differences between the time-series are due to several factors, including uncertainties in the forcing time-series and the unpredictability of some interactions between the atmosphere, ocean, land surface and sea ice.
Looking at the relative contributions of these forcings to climate change over the past 1000 years, scientists have concluded:
Model estimates of variations in Northern Hemisphere temperature over the last 1000 years. All time-series smoothed using a 40-year low-pass filter. Model anomalies adjusted to have the same mean over 1500-1899. Model simulations from Bauer et al. (2003) compare two different estimates of solar activity based on radiocarbon (14C) and beryllium-10 (10Be).
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