On Sunday, the United Nations Intergovernmental Panel on Climate Change (IPCC) released its Synthesis Report that summarizes the findings of its three earlier reports on the physical science of man-made global warming, and the analyses of how to mitigate and adapt to future climate change. The report declares:
Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, and sea level has risen. …
It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.
Each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850. The period from 1983 to 2012 was likely the warmest 30-year period of the last 1400 years in the Northern Hemisphere, where such assessment is possible (medium confidence). The globally averaged combined land and ocean surface temperature data as calculated by a linear trend, show a warming of 0.85°C [0.65 to 1.06°C] over the period 1880 to 2012, when multiple independently produced datasets exist.
The report blames the ongoing 15-year pause in global warming on "natural variability," noting:
Due to this natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade).
The authors of the report evidently expect that the pause should soon end. As a consequence the report projects:
The global mean surface temperature change for the period 2016-2035 relative to 1986-2005 is similar for the four RCPs [modeled warming scenarios] and will likely be in the range 0.3°C- 0.7°C (medium confidence).
That implies that warming could increase by as much as 0.35 degrees per decade, which is nearly triple the rate the IPCC reports for period after 1951, and 7-times higher than the rate of increase it reports for the last 15 years.
The report lays out a global carbon budget with the goal of keeping temperatures beneath the 2°C threshold relative to the 1861-1880 baseline set at the UN's Copenhagen climate change conference in 2009. Humanity has already pumped the equivalent of about 1900 gigatons of carbon dioxide into the atmosphere and can only emit 1000 gigatons more if there is to be a 66 percent chance of keeping future temperature increases below 2°C. Humanity is currently emitting about 50 gigatons per year now, which implies that the remaining carbon budget would be used up in about 20 years. The Synthesis Report notes:
Scenarios that are likely to maintain warming at below 2 C are characterized by a 40% to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions level near zero or below in 2100.
So no fossil fuels by 2100. How much would cutting greenhouse gas emissions cost? The report suggests that it would reduce annual economic growth over the remainder of the century by between 0.04 to 0.14 percent (median 0.06 percent). Interestingly, the Synthesis Report observes that estimates for global annual economic losses for temperature increases of ~2.5 °C above pre-industrial levels are between 0.2 and 2.0% of income. The report adds:
These impact estimates are incomplete and depend on a large number of assumptions, many of which are disputable…. As a result, mitigation cost and climate damage estimates at any given temperature level cannot be compared to evaluate the costs and benefits of mitigation.
Well, let's do a back of the envelope benefit-cost comparison estimate anyway. The current world GDP is around $70 trillion. Assuming a baseline growth rate of 2.5 percent for the next 85 years, world GDP would be $612 trillion in 2100. So cutting greenhouse gas emissions is estimated to reduce GDP between $591 and $545 trillion by 2100. In other words, global GDP will be between 3.5 and 11 percent lower than if there was no need to mitigate future climate change. One way to think of this is that people today making an average global per capita income of just under $10,000 per year are being asked to sacrifice economic growth and development for people whose incomes will likely be over $61,000 per year in 2100.
With due humility the Synthesis Report observes…
…it is outside the scope of science to identify a single best climate change target and climate policy.
Science may not be able to tell us what to do about man-made warming, but diplomats at United Nations climate change conferences this December in Lima, Peru and next December in Paris, France aim to do just that.