A Few Things Ill Considered

This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.

Objection:

Why should we trust a bunch of contrived computer models that haven't ever had a prediction confirmed ? Talk to me in 100 years.

Answer:

Of course, given the absence of a few duplicate planets and some really large time machines, how can we hope to test a 100 year temperature projection today? Well, we can't, but does this mean that the models can not be validated without waiting 100 years? I don't thinks so.

The climate is a very complex system and our observations of it are by no means complete, even insofar as what is going on today. This is a shortcoming we need to work hard to correct, but it is also an opportunity for validating model predictions. See what the models say some measurement we have never taken should be, then go take it and compare. But before getting into that, there are actually some global temperature predictions that have indeed been validated. We can start with one of the pioneers in climate science. Over 100 years ago, in 1896 Svante Arrhenius predicted that human emmisions of CO2 would warm the climate. He used a much simpler model than current Ocean Atmosphere Coupled Global Climate models that run on super computers, of course, and he actually overestimated the climate's sensitivity to CO2 by a factor of 2. At the same time, he hugely underestimated the degree of warming, thinking CO2 would rise very slowly (how could he have ever predicted the emissions that the future held!) but this is still a pretty impressive very early success.

Running the clock forward, in 1988, James Hansen of NASA GISS fame predicted that the temperature would climb over the next 12 years, with a possible brief episode of cooling in the event of a large volcanic eruption. He made this prediction in a landmark paper and before a Senate hearing, which marked the official "coming out" to the general public of the dangers of Anthropogenic Global Warming. 12 years later, he was proven remarkably correct, requiring an adjustment only for the timing difference between the simulated future volcanic eruption and the actual eruption of Mount Pinatubo.

And let's face it, every year that goes by with an ever increasing global mean temperature trend is one more year of success for the climate models that tell us this will continue to happen until CO2 concentrations stop rising. As well, the predicted acceleration of the rise is also apparent, though to be fair decades will need to pass before confirmation of this is unarguable.

But putting global surface temperatures aside, there are some other significant predictions of enhanced greenhouse gas warming that have been made and confirmed:

• the warming at the surface should be accompanied by cooling of the stratosphere and this has indeed been observed
• as well as surface temperatures warming, models have long predicted warming of the lower, mid and upper troposphere even while satellite readings seemed to disagree. But it turns out the satellite analysis was full of errors and on correction, this warming has been observed
• models expect warming of ocean surface waters as is now observed
• models predict an energy imbalance between incoming sunlight and outgoing infrared radiation. This has been detected
• models predict sharp and short lived cooling of a few tenths of a degree in the event of large volcanic eruptions and Mount Pinatubo confirmed this.
• models predict an amplification of warming trends in the Arctic region and this is happening

And, to get back to global temperatures where we started, models predict continuing and accelerating warming of the surface and so far they are correct. It is only the long term predictions that come with severe consequences that needs the passage of time to prove or disprove them, but frankly we don't have that time at our disposal, action is required in the very near term. We must take the many successes we have already seen as the strong validation that it is.

But if we seek even more confidence, there is another way to test a model's "predictive" power over a long time period and that is called hindcasting. By starting the model at some time in the past, say the turn of the 20th century, and running it forward from that point, all the while feeding it the data about how GHG and aerosol and solar and volcanic and albedo forcing all did play out according to observation, we can directly compare modelled behaviour with actual observations. This of course has been done many times. Have a look at this page and judge for yourself how they did. Would a prediction of temperature for year 2000 made in 1900 have been validated? Would politicians in 1900 have been wise to heed the warnings of science had science been able to do this at that time? Clearly, yes.

This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.

"The Models are Unproven" was first published here, where you can still find the original comment thread. This updated version is also posted on the Grist website, where additional comments can be found, though the author, Coby Beck, does not monitor or respond there.