What would that make a 100-year forecast of global temperature?
From the transcript of James Hansen’s testimony before the House of Commons:
Q6 Dr Offord: In January this year, the UK’s Meteorological Office published its decadal forecast. They predicted that in the next five years climate change would be slowing, slower than they had predicted in 2011. What does your research show about global climate change slowing down?
Professor Hansen: Five-year forecasts for global temperature are extremely speculative. What we can say about climate change in the five-year timescale would be based mainly on the measurements I have already mentioned, which are the earth’s energy imbalance. We know there is more energy coming in so, on a decadal timescale, we know the planet is going to get warmer. We can say it with a high degree of confidence, not 100% certainty. We are not certain that huge volcanoes will not go off in the next five years. That is unpredictable. If you had huge volcanoes that put aerosols in the stratosphere to reflect sunlight away, then they would change this planetary energy imbalance, and you may have less energy coming in than going out. You cannot be certain, but we do know, from looking at hundreds of years, what the probability is that we get some forcing like that.
We also know how the sun is changing. In addition to human-made factors, there are natural factors that influence global temperature on those timescales. One of them is the sun. The sun goes through these cycles in approximately 11-year periods, which are associated with the solar magnetic cycle, and the most recent cycle is weaker than any of the prior three. We began to make very precise measurements from satellites in the 1970s.
The sun is contributing a bit to slowing down the warming because the sun’s brightness has been about one-tenth of a watt less in the last decade than in the prior decades. Despite that solar, the increase in CO2 over that decadal time period is contributing a forcing of a few tenths of a watt per metre squared, so it exceeds the reduction in solar irradiance. Therefore I can say, with a reasonably high degree of confidence, that over the next several years you will get warming and, therefore, we will see still higher records.
The warmest year on record was 2010. There has been a lot of press recently about the fact that some people say, “Oh, the warming has stopped over the last 15 years”. They are comparing to 1998, 15 years ago, when we had the El Niño of the century, which caused global temperature to jump two standard deviations above the trend line at that time. It is true that over the last decade temperatures have been only comparable to that 1998 temperature. That is a tricky statement because, if you take the 10-year running average, what you see is that the temperature is still going up, but there is variability on that short a timescale.
To summarise, we can confidently say that the next decade is going to be warmer than this decade. This decade was warmer than the one before. That has been true now since 1970-each decade has been warmer, and, because of the rapid emissions growth in CO2, that is going to continue to be the case until we slow down our emissions of fossil fuels.