Obama’s Fight against Global Warming
By Nayyer Ali MD
 
Last column I looked at what the IPCC’s latest assessment report had concluded about the effects of global warming so far.  Rising levels of carbon dioxide due to burning of oil, coal, and natural gas, have caused a rise in global average temperatures (GAT).  This rise will continues as long as we burn fossil fuels for energy.  But we can mitigate and reduce future damage by reductions in fossil fuel use.  President Obama has done three major actions on this front. 
First, he got automakers to accept a rise in average MPG for new cars to 55 in 2025, from a current 27 MPG.  Second, he is using the power of the EPA to demand a sharp cut in CO2 emissions by power plants over the next 15 years.  And finally, he got China to agree to vastly expand its use of renewable energy and to commit to a peak in fossil fuel use by 2030.  But what is needed is a more ambitious global compact on energy. 
The current state of the science finds that expected rise in temperature per 1000 gigatons of carbon (gTC) emitted (called the Transient Climate Response) is between 0.8 and 2.5 degrees. The TCR is a lower value than the ultimate change in temperature after several more decades as the climate system reaches a new equilibrium.  
The IPCC stated that a CO2 concentration of 430-530 ppm would be consistent with keeping GAT rise to less than 2 degrees compared with 1900 (we have already risen 0.85 degrees, and current CO2 concentration is at 400 ppm, up from 280 in 1900).   Concentration between 530-580 ppm consistent with a 2.5 degree rise.  580-720 ppm consistent with 3 degree rise.  720-1000 ppm would be consistent with 3.5 degree rise.  
To model the future, the IPCC created four scenarios, labeled RCP 2.6, 4.5, 6.0, and 8.5 which designate how much greenhouse effect the planet experiences.  Each scenario makes predictions about global population and economic growth, policy choices, intensity of fossil fuel use, relative price of renewables, etc.  The lowest emission pathway is 2.6, with the others representing higher and higher carbon emission scenarios.  Only RCP 2.6 is consistent with the goal of keeping GAT rise to under 2 degrees, though the others show varying rises.  
In RCP 2.6 temperatures rise from current by 1 degree +/- 0.5 degrees by 2100.  Arctic summer sea ice continues to diminish over the next few decades but stabilizes and levels off around 2050.  Sea levels rise 40 cm (16 inches) and ocean pH drops from 8.1 to 8.05.  To achieve these goals with 67% probability emissions need to be limited to 280 gTC going forward, with 33% probability they need to be limited to 390 gTC. Concentration of CO2 in the atmosphere is expected to peak around 2070 at 500 ppm then decline to under 450 ppm.   In next warmest RCP 4.5 GAT will rise from current level by 1.4 degrees +/- 0.5 degrees by 2060, and by 1.8 degrees +/- 0.7 degrees by 2100.  CO2 reaches 550 ppm by 2100.   In RCP 6.0 temps expected to rise by 1.3 degrees +/- 0.5 degrees by 2060, but by 2.2 degrees +/- 0.8 degrees by 2100.  CO2 reaches 750 ppm by 2100.   In the hottest RCP 8.5 temps to rise by 4 degrees +/- 1.0 degrees by 2100 and 8 degrees +/- 4 degrees by 2300.  Obviously the predictions out to 2300 have a large confidence interval.  Sea level is expected to rise by 60 cm by 2100.  By 2500 sea level expected to rise between 2 to 6 meters.  Ocean pH will decline sharply from 8.1 to 7.75.  Arctic summer sea will totally melt by 2060.  CO2 concentration reaches 1000 by 2100, but rises further to 2000 by 2200 (this scenario assumes high level of fossil fuel use for 200 years).
Of note, in the relatively near term, GAT is expected to rise about the same amount in all scenarios through 2035, somewhere between .3 and .7 degrees.  A clear separation of temperature trends will start to become evident around 2035 in the RCP’s.  
If the goal is to limit warming to 3 degrees compared with 1900 (2.2 degrees compared with present-day temperatures), then this would require total further carbon emissions to be held to under 625 gTC for 67% probability of success.  For 33% probability of success, emissions need to be limited to under 800 gTC.  In all scenarios expect RCP 2.6, further warming occurs after 2100 though the amount varies by scenario.
With regard to pace of reduction in RCP 2.6, emissions peak in 2020, drop by 50% by 2050, and go mildly net negative by 2070 (mostly through reforestation and possibly carbon capture technologies that have not been invented).    
In RCP 4.5, emissions peak around 2040, and drop to 50% below current values by 2100.  In RCP 6.0 emissions don’t peak till 2080, and in 8.5 they peak in 2100 and then stay level through 2200.  
In terms of costs and pace of matching the RCP 2.6 scenario, IPCC states that this is much more likely if annual emissions in 2030 are kept below 14 gTC (currently we are at 9.5 gTC).  If emissions exceed that value then the rate of emission reduction in 2030-2050 will have to be substantially faster and therefore more costly to achieve.  
The IPCC also makes the point that to achieve RCP 2.6 pathway, emissions in the power sector will have to decline very sharply, with a 90% decline achieved by 2040-2070 time frame.  Energy efficiency is promoted as a key mitigation strategy.
The problem with these four scenarios is that they are guesswork at best and it is unknowable exactly what the trajectory of CO2 emissions will be.  Based on the current rapid rollout of wind and solar, and rising efficiency of cars, it would be safe to say that we will not go beyond 800 gTC emitted in the rest of the century.  The question is how low can we push it.  I think it is more realistic to assume that we will use more fossil fuels in the next 25 years than RCP 2.6 envisions, but we will likely stop using fossil fuels entirely by 2070, a more rapid decline than is projected in RCP 4.5.  It is also unknown how much warming that will lead to, with a range of 1 to 3 degrees possible. 
Governments should look to set overall global emission targets at the upcoming UN Climate Summit.  It would be unfair to force developing countries to give up cheap fossil fuels while they are still poor, so the burden should fall more heavily on the wealthy.  But in the long run, without the entire planet working together, even if the US and Europe dropped to zero emissions, we would still have massive fossil fuel use in 2050. 
One pathway that makes sense to me is to create national limits of carbon emission based on population.  Currently the US emits 5 tons of carbon per person, while China emits a bit over 2, and many poor countries less than 1.  The average for the globe is about 1.4 tons per person.  Countries should agree to limit emissions in 2035 to 2 tons per person, reducing that to 1 ton by 2050 and zero by 2070.  Some countries will use less than their quota while others may need to use more, and a system of trading permits can be created where countries can buy and sell unused quotas.  Meanwhile, we will need to keep a close eye on global temperature trends, if they start running hotter than anticipated we could move up the quota goals, and if trends are more benign, we could relax them.

 

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