NOAA NCDC National Climatic Data Center
NOAA Paleoclimatology Program, NCDC Paleoclimatology Branch  
Paleoclimatology Navigation Bar Bookmark and Share
NOAA National Environmental Satellite, Data, and Information Service National Oceanic and Atmospheric Administration NOAA National Climatic Data Center U.S. Department of Commerce Paleo Home Data Paleo Projects Paleo Perspectives Education and Outreach About Paleo Program Site Map GW Home Story Data End Site Map Global Warming Spanish Page Contact NOAA Paleo

Climate Model Simulations of the Last 1000 Years

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 Results

Simulations 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:
  1. Solar and volcanic forcing have been responsible for some of the variations in Northern Hemisphere temperature over the past 1000 years.
  2. Neither solar nor volcanic forcing can explain the dramatic warming of the late 20th century. Changes in these forcings during the 20th century would actually have resulted in a small cooling since 1960.
  3. Only by adding the human-caused increase in greenhouse gas concentrations are the models able to explain the unprecedented warmth of the late 20th century.
model images
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).

Back to ... Paleoclimatic Data for the Last 2000 Years

Dividing Line
Privacy Policy information Open Access Climate Data Policy link USA logo Disclaimer information
Dividing Line
Downloaded Wednesday, 18-Jan-2017 19:14:44 EST
Last Updated Wednesday, 20-Aug-2008 11:23:45 EDT by
Please see the Paleoclimatology Contact Page or the NCDC Contact Page if you have questions or comments.