What the IPCC report tells us
Here’s our interview with Pierre Friedlingstein – one of the lead authors on the latest Intergovernmental Panel on Climate Change (IPCC) report – which we were unable to fit into our October programme on Solutions. Here he explains some of the key findings of the IPCC’s fifth assessment report on the physical science of climate change.
What temperature rise have we seen to date?
If you look across the entire 20th century it’s almost 0.9 degrees [centigrade] of warming.
And what impacts have we seen in the physical world over that period due to that rise in temperature?
We’ve seen impacts in loads of quantities, like sea ice which is shrinking and is observed with accuracy over the last 30 years or so. Snow cover in the Northern hemisphere is also decreasing. Glaciers are melting across the world. Regions with intense precipitation are getting more precipitation and arid zones are getting drier. There are also some changes in extremes with more heatwaves, less cold nights and more warm nights.
Four new future scenarios have been modelled for this new report. Can you explain why these were needed and what they set out to show?
Future predictions are always based on scenarios for the IPCC. The new scenarios are called Representative Concentration Pathways or RCPs. They are dated and done with new models.
There is one scenario, the RCP2.6, with strong mitigation [ie action to avoid climate change]. In this scenario, future emissions start to decline in the near term, go down to zero and are negative by the end of the century. This scenario is compatible with a warming of two degrees.
The other scenarios – RCP4.5, RCP6 and RCP8.5 – continue emitting CO2 in the future at different rates. RCP4.5 stabilises later on, whereas RCP6 and RCP8.5 don’t stabilise. RCP8.5 is more like business-as-usual with emissions increasing across the 21st century.
For the most optimistic scenario RCP2.6 the warming across the 21st century is anywhere between 0.3 and 1.7 degrees centigrade in addition to the one degree we’ve had already. And if you take the highest scenario [RCP8.5], the warming is from 2.6 to 4.8 degrees over the 21st century on top of the 0.9 degrees centigrade we’ve had already.
From the perspective of anyone’s who’s interested in preserving a climate that is hospitable to most species on the planet, we’d be most interested in the lowest one.
Can you describe what happens to greenhouse gas emissions under this scenario? How does that play out over the century?
It does start declining soon, I think by 2020 or something, 2030 maximum. Then both CO2 and non-CO2 [greenhouse gas] emissions all decline strongly over the next fifty years. Then for CO2 they are negative after 2080 or something which means we would need to remove CO2 from the atmosphere [beyond that point]. [Note: Here’s a useful summary of the RCP2.6 scenario. The authors of the original RCP2.6 paper acknowledged that more research on low-emissions scenarios are needed. It includes a number of assumptions that might need challenging. See also the “Cleaner, Fairer Future” set out in the Two Energy Futures report that we covered in last month’s programme.]
In the latest report you’ve introduced the idea of carbon budgets for the first time. Why did you feel it was important to do that?
One of the reasons was because we could do it now and we couldn’t do it before. The climate models now take into account not only the climate system but the carbon cycle system. You can these models force with emissions of CO2, calculate the ocean and land carbon sinks, then derive how much CO2 stays in the atmosphere for a given amount of emissions. Therefore you can calculate the trajectory of CO2 and climate for a given amount of emissions. And likewise if you impose a concentration [of greenhouse gases in the atmosphere] you can back-calculate what emissions you need to have. So there is a direct link between emissions and concentration, and between concentration and climate change. This is something we did not have in the previous assessment.
So when you put all these pieces together you can compute the warming you get for different levels of emissions. When we do so we find it is more or less linear. So if you emit 500 gigatonnes [Gt] of carbon – which is more or less what we’ve emitted so far – you have warming of one degree [centigrade]. If you emit 1000 then you get a warming of two degrees, if you emit 2000 you get a warming of four degrees. So it’s pretty much linear.
And it tells you for example that if you want to limit warming to two degrees [of global average temperature] compared to [the] pre-industrial [average temperature] you are left with a total budget of something like 1000 Gt of carbon across the entire time period. Knowing we’ve emitted already 500 Gt you are left with 500 Gt of carbon you can emit in the future.
And that’s in a CO2 only world. If you take into account the fact that some of the warming would be due to non-CO2 gases like methane or N20 (nitrous oxide) the amount of CO2 you can emit is lower. We calculate that to be likely to stay below two degrees you can only emit 800 GT of carbon in total. We’ve emitted 500 already so there’s another 300 GT you can emit in the future. Currently we emit 10 GT per year so 300 GT is about another 30 years of emissions at the current rate. Beyond that it is likely we will end up with a warming which is above two degrees. That is new information which we thought was policy relevant, because you can make a direct link between future emissions and warming.
Do you think carbon budgets should now play a role in the international negotiations?
Well in a sense it was always known but it was not shown in such an obvious way. It was known for some time that you need to reduce emissions in the future to stay below a given target. From a policymaker’s point of view I guess it’s useful to know that there is one number which is like the envelope of what you do in the future but still you need to distribute this in time and you need to distribute this across countries.
With the distribution in time, you bring in the question of how fast you can decrease emissions. Obviously declining emissions all the way down to zero by tomorrow is impossible so there is a maximum mitigation rate that is proposed by [IPCC’s] Working Group III, which is something like 5% per year declining emissions. More than that is not sustainable according to their models.
Also there is the issue of which country is going to reduce first and which countries will have more time before they start reducing. These questions are hot on the agenda but they are highly political, not so much scientific. For the science, it’s obvious: every country should decline as soon as possible. [See our interview with Mohamed Adow of Christian Aid on how to distribute the remaining safe emissions budget using already agreed principles.]
Can I ask you a final question, which is outside the science? Do you feel there is a disconnect between the signal that’s coming from the science community and the response that we are getting from policymakers? If so, how do you feel about that?
I’m happy to answer this question but not as an IPCC author. My personal view is that there is a disconnect. I’m hoping that this new [IPCC] assessment report will help to recreate momentum so that countries come together and move forward. As we all know there’s not much progress since Copenhagen. There was a lot of hope but it didn’t materialise. If you look at the cumulative emissions of CO2 they’re actually increasing faster than ever. There’s no sign of declining emissions, or levelling off and starting to decline in the near future which is what you need if you want to stay below two degrees. So the two degrees target seems to me harder and harder to get. It’s not impossible but it will require lots of work and more drastic action in the future.
The sooner you start declining the easier it is. The more you wait the stronger the reduction needs to be in the future, because what matters in the end is the total [emissions]. So if we wait another thirty years to reduce emissions, then we have to cut to zero after thirty years which we know would be very complicated. So the sooner you start decreasing the easier it is in the long term. So yes, I agree that there is not much time and the world is not heading in that direction at the moment.
- See also Climate Change Roundtable – four top scientists’ views on IPCC report
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