The recent announcement of even tighter CO2 emission standards for power plants by Obama has been met with outrage by the usual suspects. This is a necessary action as the Republicans have decided to deny that global warming is even a thing. There is clearly no point in trying to get legislation on this.
I have occasionally discussed global warming at this site, but it is a touchy topic and sometimes views are expressed that are not what the scientific consensus says. While the right denies the issue entirely, there is also some inflation of the problem that I will find myself disagreeing with. As such I decided to go to the source document for the actual scientific consensus, which is the Fifth Assessment Report of the IPCC released in 2014. I have read through the 65 page Climate Change 2014 Synthesis Report that lays out the IPCC view, from which I have created this summary. I do not offer any of my own opinions, but I do condense certain points (for example, giving the expected value but not the full range for a parameter) for brevity. If anyone believes that I have erred in my summary I will try to correct in the comments, if you can point out the exact item. For clarity all temps are in degrees centigrade. Carbon emissions are in gigatons of carbon, rather than in gigatons of carbon dioxide, makes accounting easier. To convert the weight of carbon is 27% of the total mass of carbon dioxide. To see the IPCC report go to http://www.ipcc.ch/...
Past the squiggle for the details.
The IPCC begins by stating the warming in the last century has been unequivocal and unprecedented in the historical record. In the Northern Hemisphere the period 1983-2012 is very likely the warmest in the last 800 years, and probably the warmest in the last 1400 years. From 1880 to 2012 the global average temperature (GAT) has risen 0.85 degrees C. In the last 15 years the temperature has been rising at a rate of 0.05 degrees per decade. The recent rate of rise is very sensitive to the chosen start year (97 vs 98 vs 99), this slow rate of rise in the last 15 years is most likely due to natural variability and does not mean that global warming has slowed.
In the last 100 years, 90% of excess heat has been stored in the oceans, 9% has ended up in the land and ice masses. Only 1% has ended up in the atmosphere. Average ocean temps have been rising by 0.11 degrees per decade since 1970.
Since 1990 there has been loss of ice mass in glaciers, Greenland ice cap, and the Antarctic ice cap. Maximum snow extent in the Northern Hemisphere in late winter has declined by 1.6% per decade since 1967.
Sea levels have been rising for 2000 years. The rate of sea level rise has accelerated since 1900, with a rise of 19 cm in the last hundred years (roughly 5 inches). In the last interglacial about 100,000 years ago, sea levels peaked between 5 and 10 meters higher than current, though average global temperatures were at least 2 degrees warmer than current for several thousand years.
With regards to the so-called “pause” in global warming, referring to the relatively slow rate of warming since about 1997, the IPCC states that they have “medium confidence” the pause is due to natural variability. They do note that of 114 climate simulations, the expected rise in temperature in the last 15 years was higher than the actual in 111 of the simulations. When looking at longer time periods, the models and actual values line up pretty well.
CO2 levels are rising at 2 ppm per year, other greenhouse gases particularly methane and nitric oxide also rising. CO2 persists in the atmosphere for many centuries, methane lasts 12 years and nitric oxide about 120 years. In a change from the Fourth Assessment Report, IPCC now concludes that aerosols have less of a net cooling effect than previously thought.
Since 1750, humans have emitted 560 gTC (gigatons of carbon), with annual emissions now running 9.5 gTC. The main drivers of increasing annual emissions are population growth and economic growth, though the IPCC does not assign relative responsibility.
The climate consequences of warming include increased frequency and severity of heat waves and precipitation events. In general, current wet regions are expected to get wetter, while current dry regions expected to get drier. Monsoonal systems (critical to agriculture and life across East Africa, South Asia, and East Asia) are expected to enlarge. Warming will not be uniform. Warming will be greater in the polar regions and also over land rather than ocean.
The risks of global warming are to both human civilization and the biosphere. For humans extreme weather events may cause significant disruptions. Poorer nations and regions will be impacted more severely due to lack of resources to mitigate effects. There will be significant costs associated with adaptation to or mitigation of global warming. There will be both increased water scarcity in arid regions, perhaps leading to conflicts, and increased incidence of flooding events.
Extinction rates are expected to increase but IPCC did not quantify this. Fisheries are expected to deplete in the tropics and increase in the temperate zones. Coral and polar ecosystems are at particular risk. Coral may be damaged by significant changes in ocean pH, while warming may substantially shrink or eliminate Arctic summer sea ice. Impacts will be substantially larger if warming exceeds 4 degrees. With that level of warming, the entire Greenland ice sheet could melt, though the melting process will take greater than 1000 years. The IPCC concluded at this point there is no evidence of an Arctic sea ice “tipping point” when loss of albedo in the summer results in runaway melting and complete loss of the ice cap.
Cuts in greenhouse gas emissions can mitigate these risks. The current state of the science finds that expected rise in temperature per 1000 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). 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. The reasons for these labels was not given. 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 emissions 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 (10 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 assume 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 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.