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How do we know human activities are contributing to global warming?

Some of the strongest evidence to support the view that human activities are causing the Earth to warm comes from numerical experiments performed with state-of-the-art global climate models. These models encapsulate the current understanding of the physical processes involved in the climate system, the interactions, and the performance of the system as a whole. They have been extensively tested and evaluated using observations. Today's best climate models are now able to reproduce the climate of the past century, and simulations of the evolution of global surface temperature over the past millennium are consistent with paleoclimate reconstructions.

As a result, climate modelers are able to test the role of various forcings in producing the observed changes in global temperature. Forcings imposed on the climate system can be natural in origin, such as changes in solar luminosity or volcanic eruptions, or human-induced, such as increases in aerosol and greenhouse gas concentrations in the atmosphere.

Climate model simulations that account for such changes in forcings have now reliably shown that global surface warming of recent decades is a response to the increased concentrations of greenhouse gases and sulfate aerosols in the atmosphere. An example, from a climate model simulation performed at the National Center for Atmospheric Research (NCAR), is provided in Figure 1. When the model is integrated forward in time over the 20th Century with only information on imposed natural forcings, there is no discernible trend in global surface temperatures over the last several decades (blue line). When changes in greenhouse gas and aerosol concentrations are added to these natural forcings, however, the model not only simulates an increase in global surface temperature (red line), but it almost exactly reproduces the observed rate of change (black line). Numerous simulations for each case are run, and the solid lines represent the mean while the shaded regions indicate the "spread" about the mean. This spread reflects intrinsic natural climate variations arising from purely internal atmospheric processes as well as from interactions among the different components of the climate system, such as those between the atmosphere and oceans or the atmosphere and land.

Such results, which have also been produced by several other independent modeling groups, increase our confidence in the observational record and our understanding of how global mean temperature has changed. They also indicate the time histories of the important forcings are reasonably known, and the climate processes being simulated in models are adequate enough to make the models very valuable tools for investigating the causes and processes of past climate variations as well future climate change.