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Fifth National Climate Assessment - Read the Report

Exploring human and natural influences on climate

Model runs of CMIP5 models without greenhouse gas forcing (orange lines) demonstrate natural variability in average annual global surface temperatures expected without human influence on the climate. Model runs that incorporate greenhouse gas forcing due to human activity (red lines) are consistent with observed global surface temperatures 1880–2016 (black line). Source: NOAA.

Earth system models allow researchers to evaluate the size and strength of various influences on the climate system and identify the human contribution to the warming trend.  

Earth system models allow researchers to distinguish “internal” climate variability (natural climate cycles) from the effects of “external” influences on the climate, both human and natural (including variations in incoming solar energy, volcanic eruptions, and greenhouse gas emissions from human activities). Model simulations of natural variability from the late 1800s to the present (see figure, orange curves) do not match observed warming (black curve), indicating that external forces are influencing the climate system. To test how different external influences on the climate have affected global temperatures, models participating in the DOE-sponsored World Climate Research Program's Coupled Model Intercomparison Project 5 (CMIP5) were run using estimates of natural factors (changes in incoming solar energy and volcanic aerosols) and human factors (greenhouse gas emissions from burning fossil fuels, land use change, and aerosol pollution).

Taking into account all known external influences on the climate, the average global temperature change across model runs (red curve) tracks closely to actual observations (black curve)—indicating that the CMIP5 models can reproduce global climate processes fairly accurately. These comparisons support and strengthen the conclusions of the Intergovernmental Panel on Climate Change Fifth Assessment Report and the Third National Climate Assessment attributing at least a substantial part of the observed warming to human activities, with average temperature change showing a significant departure from the limits of natural climate variability consistently after 1980.

CMIP5 models include Earth system models from across the globe, the Community Earth System Model funded jointly by NSF and DOE, the Goddard Institute for Space Studies Model funded by NASA, and the Geophysical Fluid Dynamics Laboratory Model funded by NOAA.