Source: BBC Radio 4: Today Programme
URL: http://bishophill.squarespace.com/storage/Hansen%20on%20Today%20170513.mp3
Date: 17/05/2013
Event: Hansen - understanding of climate sensitivity "based on the Earth's history, not on climate models"
Credit: BBC Radio 4, also thanks to Andrew Montford (Bishop Hill) for the audio link
People:
Sarah Montague: Why isn't the world getting warmer? Or at least as quickly as most climate scientists expected? We're still chucking out carbon dioxide but, for some reason in recent years, it's not been reflected in higher temperatures. The UK Met Office has revised down its forecasts, and although it says the long-term trend is still for warming, the global average temperature will have stayed roughly the same for two decades. Professor James Hansen is Adjunct Professor at Columbia University's Earth Institute. He's also outgoing Director of NASA's Goddard Institute for Space Studies - he's just recently announced his retirement, and he's here with us in the studio. Good morning, Professor Hansen.
James Hansen: Hi, good morning.
Sarah Montague: How do you explain what's going on with temperatures?
James Hansen: Well, I should correct what you just said. It's not true that temperature has not changed in two decades. In the last decade, it's warmed only about a tenth of a degree, as compared to two tenths of a degree in the preceding decade. But that suggests natural variability - there's no reason to be surprised by that at all.
Sarah Montague: But there was a suggestion that we should have been expecting 0.2 of a degree. And it see- and it has -
James Hansen: No, if you look over a 30 or 40-year period, then the expected warming is about two tenths of a degree per decade, but that doesn't mean that each decade is going to warm two tenths of a degree - there's too much natural variability. And there are other forcing factors - it's not only CO2 that's changing. The Sun's brightness changed - decreased slightly, over the last ten years, about a tenth of a watt per metre squared. That's not as large as the climate forcing that you get from increasing CO2, but it's not negligible. In addition, China and India have been pumping out aerosols, by burning more and more coal, so you get from that not only CO2 but also these particles that reflect sunlight and reduce the heating of the Earth. So there's nothing - it's a complicated system, but there's no change at all in our understanding of climate sensitivity and where the climate is headed.
Sarah Montague: So the UK Met Office revising down its forecast for temperatures - are they wrong or are we misreading what they're doing, and this is a temporary adjustment and it'll be revised up again?
James Hansen: Ah - our understanding of how sensitive the climate system is, is based on the Earth's history, not on climate models. So we have good data on how the Earth responded in the past when carbon dioxide changed, and other forcing factors changed. So there's no reason to change any forecast for the long term.
Sarah Montague: So the Met Office is wrong...
James Hansen: Well, if you want to look at the next decade or two, then you should take account of the fact that the last decade only warmed one tenth of a degree, rather than two tenths of a degree. So in that sense, of course they're using the empirical information of what the starting point is today. But -
Sarah Montague: But you, are you - I mean, I take your point. You're looking back at the way - you say we know how the Earth responds. But there are an awful lot of things we do not know, and one of the suggestions, in fact our correspondent Roger Harrabin was making in his report an hour ago, is actually what the oceans are doing. That there are various suggestions - one suggestion is actually: the whole world is responding, it's absorbing, it's adapting and therefore the consequences of our action or the warming will not be what we thought. The other is that there's this possible delay because the oceans are -
James Hansen: No, not at all. You see, if we look at Earth's history, we get the event- we understand what the eventual response will be. Now how it tracks over a period of time, a decade or a few decades, does depend upon how fast the ocean is taking up heat, for example. But these are really details, and this is a diversionary tactic. Our understanding of global warming and human-made climate change has not been affected at all. You know, I didn't come to Europe to talk about details which are very technical details -
Sarah Montague: So the fact that the last decade is 0.1 -
James Hansen: No, I -
Sarah Montague: - is neither here nor there in the great scheme of things.
James Hansen: Well, you know - do you know that Canada is in the process of trying to get Europe to agree that tar sands are no different than conventional oil, even though they put much more carbon dioxide into the atmosphere? And they're hoping to twist the arm of the United Kingdom, France and other European countries and get them to agree to pretend that tar sands are no more damaging. If we don't wake up to that, if you don't wake up to that, then we're going to put our children in a position where they have climate change ongoing that's out of their control.
Sarah Montague: Can I just ask you, can you understand why people listening - outsiders, non-specialists - hear both sides of the debate and feel that they may be over-egging it for their own reasons?
James Hansen: Uh, well, I understand why - it's because the deniers want the public to be confused. They raise these minor issues, which are something between experts, and then we forget about what is the main story. The main story is: carbon dioxide is going up, it's going to produce a climate which is going to have dramatic changes if we don't begin to reduce our emissions. And we can't introduce into this problem the dirtiest fuels on the planet.
Sarah Montague: Professor James Hansen, thank you very much.
James Hansen: Thank you.