3.4 Climate projections  (Page 6/8)

In addition to predicting warming of the atmosphere, climate models also suggest that sea level will continue to rise. Since 1880, sea level has risen 20 cm (approximately 8 inches) as seen in Figure Sea Levels since 1880 . This rise has been primarily the result of the water thermally expanding as it warms along with the atmosphere. Polar ice cap melt from land-based ice sheets and glaciers has also added to increase in sea level. The current projection is that sea level will rise at the rate of at least 2 mm per year over the next century, with an overall increase ranging from 15 to 60 inches cm.

How much confidence can we place in predictions about temperature and sea level by climate scientists? Let’s take a little detour before we address this important question directly. Imagine you are contemplating signing up with a psychic so you can better plan for the future—why save for retirement if money is tight and you’re not sure how long you’ll live? But you are uncertain about whether she can really see what lies ahead. You could pay her $20 a week for her predictions, and discover over time whether they come true or not. The trouble is, during this trial period you wouldn’t know whether to spend your money as fast as you make it or put some aside. But you come up with a better plan. You’ll pay the psychic $20 one time, but instead of asking her to predict your future, you’ll ask her to tell what has happened to you in the past week. If she gets that right, she gets your business.

Along similar lines, climate scientists assess the trustworthiness of their models by checking how well they “predict” the past. In Figure Model Simulations , 58 different climate model simulations were tasked with predicting the past climate from 1900 to 2005. By comparing the model simulations to the observed temperature record the scientists with the IPCC tested the accuracy of their models. In Figure Model Simulations , the yellow lines in the top panel trace out the individual model simulations, the red line shows the model ensemble mean, and the black line represents the actual observed mean temperature. The models performed exceedingly well, as evidenced by the very small variability around the observed temperature. The success of this test demonstrates the high-quality construction of these models and shows they are capable of accurately projecting the earth’s future climate.