Paris Agreement: Science points both ways

You could forgive anyone for being confused.

Two weeks ago, scientists were all over the news saying it’s going to be easier to meet the Paris Agreement targets than previously thought. Last week, they (well, not the same ones, obviously) were saying the opposite.

Could science please make up its mind?

There’s going to be a lot more of this over the next year or so.

When governments included (against expectations) the goal of keeping global warming below 1.5 Celsius in the Paris Agreement, they also asked the Intergovernmental Panel on Climate Change (IPCC) to write a special report detailing what a 1.5ºC goal means. It's coming out next year.

But much of the science it requires hadn't been done, because a 1.5ºC goal previously seemed politically unrealistic. Now, scientists are busy supplying the necessary analysis and putting it into the scientific literature.

Added to which, as the IPCC report comes closer, any press officer worth his or her salt will think about using the Paris Agreement framing as a way of stimulating interest in new bits of research.

So – prepare for your glass to be half-filled and half-emptied again on a regular basis.

When trees go bad

Last week’s two papers were quite straightforward in their aims and conclusions.

One re-calculated emissions of methane, the second most important greenhouse gas, from cattle around the world, and found it’s about 11% higher than previously estimated.

Numbers of cattle are rising; but also, breeding programmes are producing bigger animals which burp out accordingly bigger volumes of methane. This is probably one of the reasons why the concentration of methane in the atmosphere is rising again.

Last week’s other paper other looked at forests – historically a great absorber of carbon dioxide from the atmosphere, and so a natural brake on man-made climate change.

Those days may be over, these scientists found – the latest in a number of papers to suggest that globally, forests now emit more CO2 than they absorb.

That’s partly due to impacts of climate change – warming and drying - and partly due to degradation of forests through logging, burning and other activities.

The big caveat on both of these papers is that it’s a lot harder to make precise measurements of emissions (and indeed absorption) in forests and farming than in industry or transport. But… there it is.

And yes, both findings, if confirmed, will indeed make it harder to achieve the Paris Agreement's 1.5ºC goal. Unless you take the view that it's easier for humankind to reduce methane emissions from cattle than it would be to curb methane release from melting permafrost, were that the cause of rising concentrations.

Model argument

The paper that emerged the week previously is a very different beast. Herein, a team of mainly British researchers recalculated the maximum ‘carbon budget’ for 1.5ºC.

It created a mini-storm both inside science and in the media, with interpretations varying from ‘oh good, it makes the Paris Agreement more achievable’ to ‘scientists admit to having exaggerated global warming because climate models don’t work’.

The cause of that confusion mainly lies in the way it was presented to the media, although there’s a fair bit of confirmation bias at work as well. But, all of that matters much less than what the paper actually says.

Its main conclusion is that the ‘carbon budget’ for 1.5ºC – that is, the maximum amount of CO2 we can emit for a good chance of keeping global temperature rise to 1.5ºC – is considerably larger than previous estimates. The old estimates were roughly equivalent to seven years of global carbon emissions at current levels, as of the beginning of 2015 - the new one to about 20 years.

While it’s entirely understandable that some reports drew the ‘models are wrong’ conclusion given how the research was presented, the paper doesn’t show that. It shows the reverse: the amount of surface warming since pre-industrial times is within the range projected by the models. Towards the low end, for sure – but within range.

If this new carbon budget isn’t down to rubbish models, then why is so much higher than previous estimates?

There are three key reasons. One is that this team looked at the issue with considerably more precision. Given that 1.5ºC isn’t far away, they decided to begin where we are now – well, 2015, which is pretty much ‘now’ – rather than using projections starting way back when. It’s like switching from telescope to microscope. It wouldn’t make much difference to the outcome if you were dealing with a target way off in the future, but it makes a big difference when the target’s already looming on the horizon.

So: the amount of warming since pre-industrial times is 0.93ºC, the study says. That means there’s about 0.6ºC left to go, and the budget for that works out at about 200-240 gigatonnes (Gt) of carbon – about 20 years, at current rates of emissions.

The second reason is that they used a very precise definition of ‘temperature rise since pre-industrial times’. The lay meaning of that phrase is pretty obvious, but if you’re looking for something very precise, there’s a huge range of choices available.