3.4 degrees by 2100

Source :NOAA

By approaching 2100, a world set for 3.4˚C will, on present trends, probably be the reality confronting our descendants – slightly less warm than looked likely a year ago, analysts think. That’s the good news, you could say.

But the bad news is twofold. First, this improvement in planetary prospects will still leave the global temperature increase more than twice as high as the internationally agreed target of 1.5˚C. And secondly, it depends largely on the efforts of just two countries – China and India.

They have made significant progress in tackling climate change in the last twelve months. In contrast, a report by the analysts, from the Climate Action Tracker (CAT), says that not only US climate policy has been rolled back under President Trump. Most individual governments’ climate commitments are going in the wrong direction.

The CAT report says the world will – on present trends – still reach 2100 a long way above the 1.5˚C target for the Earth’s maximum tolerable temperature rise, which was endorsed in the Paris Agreement.

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These guys have done the thorough analysis, so I won't argue with their depressing conclusions.  But I will explain how I come to a lower number.

World temperatures are rising by 0.2 degrees C every decade, on average.  So if that rate continues, then by 2100, 8 decades from now, the increase will be 1.6 degrees C added to what we already have experienced (+1.5 C according to Berkeley Earth).  That gives 3.1 degrees, implying that these analysts expect the decadal increase to rise slightly, which would be perfectly consistent with ongoing rises in atmospheric CO2.  (However, BEST's calculation of the increase in global temperatures is higher than other bodies because they go farther back, and the article doesn't say whether they're using BEST or other calculations to estimate how much temps have risen since pre-industrialisation.  But since the 1970s, global temperatures have risen about 1 degree C)

I come to a more optimistic (but still not very happy) forecast based on estimated timetables for the transition of electricity generation to renewables and the electrification of transport. At some point in the mid 2020s, existing coal power stations will start being closed because renewables plus storage will start getting or will already be cheaper than the operating costs of coal power stations.  That shift will prolly take (let's be conservative) 20 years.  This will reduce global CO2 emissions by 30%.  With transport, EVs and PHEVs will likely reach 100% of total car sales by the late 2020s.  It will take another 20 years for the global car fleet to be 100% electric. 

So by 2050 annual emissions could have fallen 60%.  This isn't enough to stop the level of CO2 in the air from rising.  For that to stabilise, we need to cut emissions to below the annual amount which is withdrawn from the atmosphere by natural processes, ignoring any man-made efforts to sequester carbon dioxide safely out of the atmosphere (carbon capture and storage, or CCS), or by at least 80%.  Forest clearing and burning contributes 10%, cement 5-6%, iron and steel 4-5%, air travel 3%.  We could stop destroying forests and even start reforestation--which some scientists estimate might itself move us one quarter of the way needed to limit the temperature rise to 1.5%  Stopping forest clearing and burning will by itself take emission reductions to 70%, starting a global program of reforestation will get us even closer.  Green concrete will help, as will steel production processes which use more renewable energy, but we will prolly need to have (and pay for) CCS for cement and steel.  Air travel will be partially electric or will use fuel created via the Sabatier process.

To sum up, I see net CO2 emissions falling by 80%+ by 2050.  Until 2050, temperatures will keep on rising by 0.2 degrees C per decade, or another 0.6 C.  But after that, as atmospheric CO2 peaks, the decadal  temperature increase will slow.  And none of this will be happening in a vacuum.  As temperatures rise and droughts, floods and storms worsen, the political pressure will increase. Within countries, politicians in the pay of fossil fuel interests will lose office.  Between countries, slow movers will be pressured to up their games.  I don't think we'll see 3.4 C.  But we will almost certainly see 1.8 C (from the 1970s) by 2100.  That will be bad enough: pray we don't see 3.4 C.