Fossil Fuels Must Be Phased Out by 2100, Says UN Intergovernmental Panel on Climate Change Synthesis Report

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.