Hansen: projections and reality

It's been 30 years since Jim Hansen warned us about global warming.  "The greenhouse effect is here", he said on 23rd June 1988 to the US Senate.  What have we done since then?

The first chart below shows the three scenarios projected by Hansen back in 1988.  Scenario A has the highest growth in CO2, NO2, methane and CFCs, scenario B lower rates and scenario C projected stability, i.e., no increase in greenhouse gases. 

And at first you'll say, but hang on, his projected temperature is much higher than what actually happened.  The model was wrong!  Um, no.  Remember that to get these results his team had to project what would happen to the inputs to the model, i.e., the emissions/levels of the greenhouse gases I mentioned above.  Actual CO2 turned out to be pretty much what Hansen and co projected they would be.  But Hansen projected ongoing rises in CFCs , which are very powerful greenhouse gases.  In a rare win for co-operation and good sense, CFCs were phased out of production after the Montreal Protocol came into force in 1988.  If Hansen had factored in the levelling off in CFCs  in the atmosphere, he would have got a result quite close to what actually happened (see the second chart below).

(Source for both charts: RealClimate)

There's good and bad news from all this. 

The good news is that the world actually agreed to phasing out CFCs back in 1988, and that saved us 0.5 degrees C of global warming.   If we could agree then, we might yet be able to agree with CO2 and NO2.  Yes, fossil fuel industries now are bigger and more powerful than CFC industries were then, and yes, that well hadn't been poisoned by the Right's demented passion for coal.  But the power of coal and oil industries is dwindling.  The evidence of global warming gets every day more scary.  Most ordinary people, as opposed to politicians, are convinced that global warming is real and that we need to do something about it.  And the costs of renewables have fallen and keep on falling so precipitously that the switch to renewables is happening anyway, despite the rantings of the Right.  The opposition to a genuine climate agreement with teeth is diminishing, while the necessity for it grows every day more pressing.

The bad news is that Hansen's scenario C, which assumed that production of all greenhouse gases would stop immediately, still had temperatures rise by 0.5 degrees over the next 20 years, before slowly starting to decline.  This is because there are lags in the system.  For example, daily temperatures don't peak at midday when insolation is at its peak but a few hours later, when the sun's power is already waning.  These lags mean that even if we cut output of greenhouse gases to zero by 2050, temperatures will likely go on rising by another 0.5 degrees C up to 2070 before starting to gradually decline.  And by 2050, temperatures will already have risen another 0.6 degrees from the greenhouse gases which we will continue to emit into the atmosphere up till then.  So that means another 1.1 degree C rise in global temperatures.  The 1 degree we have had so far is bad enough.  What will another 1 degree do?

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