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How Accurate Are Existing Computer Climate Modeling Techniques?

Ask a Scientist - December 2013

T. Fulmer of Lititz, PA, asks "How accurate are existing computer modeling techniques and can we be confident that they identify human activities as the primary cause of global warming?" and is answered by UCS Kendall Fellow Roberto Mera, Ph.D.

Climate scientists have made many improvements to computer models over the years that have increased their accuracy and reliability. These advances have allowed scientists to show, unequivocally, that human activities are the major driver of global warming.

During the last five years, for example, climate scientists have made dramatic advances (pdf) in their ability to track two key climate indicators—ocean heat content and Arctic sea ice seasonal cycles—and are confident that current models can reproduce surface temperature increases since 1870, including the rapid warming in the second half of the 20th century.

There remain some uncertainties with climate model performance, but that is to be expected from any system that aims to approximate conditions in the real world. Scientists are still trying to nail down cloud processes, aerosol distribution, ocean models, and sea ice changes. But model developers and climate physicists are addressing these issues by using large numbers of model simulations as well as a variety of statistical methods based on current and past observational data.

Climate modelling and forecasting accuracy has been questioned lately due to the apparent “pause,” or “speed  bump,” in the rise of global surface temperatures over the last 15 years. In fact, climate forecasts conducted in the 1990s have been quite accurate in simulating what happened since the year 2000. A study by Myles Allen and colleagues at Oxford University, for example, compared climate forecasts that begin in 1996 with the actual temperatures observed since. They found that the simulations accurately predicted the warming experienced in the past decade to within a few hundredths of a degree.

The truth is that global warming has not paused. It is true that the rate of surface temperature warming is somewhat smaller over the last 15 years, but selectively citing the period from 1998 to 2012 is inappropriate—especially since ocean heat content continued to rise at a steady pace and the Arctic sea ice hit record lows in 2012. The year 1998 was a record temperature year, due to a strong El Niño. Shifting the time period just two years earlier, from 1996 to 2010 instead of from 1998 to 2012, the surface temperature trend increased 0.14 C per decade, slightly greater than the long-term trend. That’s why it is incorrect to focus on 15-year increments when we’re talking about temperature increases over decades, if not centuries. 

Advances in modelling also have enhanced scientists’ ability to separate natural causes from human activities’ influence on the climate system. Natural changes alone can’t explain the temperature changes that have occurred over the last 150 years. For instance, when climate models include only recorded natural climate drivers, such as the sun’s intensity, they cannot accurately reproduce the observed warming during the latter half of the 20th century. However, when scientists include human-induced climate drivers, such as carbon dioxide emissions, in their simulations, they accurately capture recent temperature increases in both the atmosphere and the oceans.

Thanks to improved climate models, scientists are now able to discern human influence on not just the general climate, but also on extreme weather events linked to climate change. Thousands of simulations generated by the climateprediction.net project have shown that heat waves such as those in Europe in 2003, Russia in 2010, and Texas and Oklahoma in 2011 (pdf) were more likely to occur in the current decade than in the mid-20th century, when greenhouse gas concentrations in the atmosphere were lower than they are today.

Regional climate modelling also has improved dramatically, allowing scientists to better predict the potential impact of global warming on specific spatial slices of the Earth’s surface. The most recent Intergovernmental Panel on Climate Change report was able to double the climate model resolution. This enhanced resolution enables scientists to better represent the natural system in areas with complex terrain, such as the western United States, and demonstrate the link between heat waves and climate change

Dr. Roberto Mera joined UCS in April 2013 as a Kendall Fellow on Climate Attribution. His work entails analyzing specific carbon emissions to determine how they are affecting global temperatures and extreme heat events.Dr. Mera's main focus and motivation is the interface between climate science and society. He received his M.S. and Ph.D. in marine, earth and atmospheric science from North Carolina State and a B.S. in atmospheric science from the University of North Carolina at Asheville.

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