Sunday, September 24, 2017

Is There Really Still a Chance for Staying Below 1.5 °C Global Warming?

Difference between modeled and observed warming in 2015, with respect to the 1861-1880 average. Observational data has had short-term variability removed per the Otto et al 2015 approach used in the Millar et al 2017. Both RCP4.5 CMIP5 multimodel mean surface air temperatures (via KNMI) and blended surface air/ocean temperatures (via Cowtan et al 2015) are shown – the latter provide the proper “apples-to-apples” comparison. (Chart by Carbon Brief) Click to Enlarge.
There has been a bit of excitement and confusion this week about a new paper in Nature Geoscience, claiming that we can still limit global warming to below 1.5 °C above preindustrial temperatures, whilst emitting another ~800 Gigatons of carbon dioxide.  That’s much more than previously thought, so how come?  And while that sounds like very welcome good news, is it true? Here’re the key points.

Emissions budgets – a very useful concept
First of all – what the heck is an “emissions budget” for CO2?  Behind this concept is the fact that the amount of global warming that is reached before temperatures stabilize depends (to good approximation) on the cumulative emissions of CO2, i.e. the grand total that humanity has emitted.  That is because any additional amount of CO2 in the atmosphere will remain there for a very long time (to the extent that our emissions this century will like prevent the next Ice Age due to begin 50 000 years from now).  That is quite different from many atmospheric pollutants that we are used to, for example smog.  When you put filters on dirty power stations, the smog will disappear.  When you do this ten years later, you just have to stand the smog for a further ten years before it goes away.  Not so with CO2 and global warming.  If you keep emitting CO2 for another ten years, CO2 levels in the atmosphere will increase further for another ten years, and then stay higher for centuries to come.  Limiting global warming to a given level (like 1.5 °C) will require more and more rapid (and thus costly) emissions reductions with every year of delay, and simply become unattainable at some point.
How some media (usual suspects in fact) misreported
We’ve seen a bizarre (well, if you know the climate denialist scene, not so bizarre) misreporting about Millar et al., focusing on the claim that climate models have supposedly overestimated global warming.  Carbon Brief and Climate Feedback both have good pieces up debunking this claim, so I won’t delve into it much.  Let me just mention one key aspect that has been misunderstood.  Millar et al. wrote the confusing sentence:  “in the mean CMIP5 response cumulative emissions do not reach 545GtC until after 2020, by which time the CMIP5 ensemble-mean human-induced warming is over 0.3 °C warmer than the central estimate for human-induced warming to 2015”.  As has been noted by others, this is comparing model temperatures after 2020 to an observation-based temperature in 2015, and of course the latter is lower – partly because it is based on HadCRUT4 data as discussed above, but equally so because of comparing different points in time.  This is because it refers to the point when 545 GtC is reached.  But the standard CMIP5 climate models used here are not actually driven by emissions at all, but by atmospheric CO2 concentrations.  For the historic period, these are taken from observed data.  So the fact that 545 GtC are reached too late doesn’t even refer to the usual climate model scenarios.  It refers to estimates of emissions by carbon cycle models, which are run in an attempt to derive the emissions that would have led to the observed time evolution of CO2 concentration.

Does it all matter?
We still live in a world on a path to 3 or 4 °C global warming, waiting to finally turn the tide of rising emissions.  At this point, debating whether we have 0.2 °C more or less to go until we reach 1.5 °C is an academic discussion at best, a distraction at worst.  The big issue is that we need to see falling emissions globally very very soon if we even want to stay well below 2 °C. That was agreed as the weaker goal in Paris in a consensus by 195 nations.  It is high time that everyone backs this up with actions, not just words.

Read more at Is There Really Still a Chance for Staying Below 1.5 °C Global Warming?

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