Would it Take to Limit Global Warming to 2°C ?
limit global temperatures from rising more than 2°C above
their pre-industrial levels, the global economy would have
to immediately cut the amount of carbon dioxide (CO2)
that it emits to the atmosphere, and sustain more concerted
cuts through the 21st Century, according to an international
team led by Glen P. Peters of the Norwegian Center for International
Climate and Environmental Research. In a commentary
they wrote1 for Nature Climate Change,
Peters compares the track record of emissions of CO2
since 1990 with four widely used sets of projections for
CO2 emissions in the future.
Emissions of CO2 have not gone down, nor has
the growth rate declined over the last several decades.
In fact, the growth rate has accelerated since 2000, by
+3.1% per year, after being +1% in the 1990s and +1.9% in
the 1980s. The amount of CO2 released has consistently
been at the high end of the range of various projected growth
curves dating back to 1990. Estimates were prepared for
the Intergovernmental Panel on Climate Change (IPCC) at
four different times (1990, 1992, 2000, and 2012-13). Each
estimate involved a projection to the year 2050 or beyond.
highest estimates in each of these four sets of projections
would lead to a global warming anywhere from +3.5° to
+6.2°C by 2100. Clearly, a global “warming cap”
of +2°C or less will require reductions in CO2
emissions from today’s rates, and some think that
the 2° goal will require “net negative emissions,”
which means that humans would directly remove CO2
from the atmosphere through some intervention.
to Peters, the goal of +2°C or less of global warming is
possible if global emissions of CO2 are cut by
3% per year, starting by 2020. If the peak rate of CO2
emissions is not achieved by 2020 before the emissions start
to decline, then the goal of limiting global warming to
2°C “may become unfeasible,” he adds.
authors cite examples of countries that cut their CO2
emissions over 10-year periods. France, Belgium, and Sweden
managed to cut their emissions by 4 to 5% per year in the
1970s and 1980s, after the oil crises of 1973 caused utilities
to shift from oil to nuclear power. Electric utilities in
the United States reduced emissions by 1.4% per year after
2005, as cheaper natural gas was substituted for coal to
generate electric power at less cost.
concludes that “unless large and concerted mitigation
efforts are initiated soon, the goal of remaining below 2°C
will very soon become unachievable.”
more substantial case for policies and pathways over the next
7 or 8 years that might succeed in limiting global warming
to 2°C over the long term was articulated in an article
in Nature Climate Change2 this year. Joeri
Rogelj and three co-authors explored how different amounts
of greenhouse gases emitted from now until 2020 will affect
the feasibility of meeting the 2°C target in the long term.
Within a range of CO2 emissions that would be permitted
in 2020, from 20% below to 6% above the current (2012) emissions3,
the option of limiting the warming of the planet to 2°C or
less is still open. The size of this "feasibility window"
depends on whether certain energy technologies (nuclear power,
biogas, etc.) can be reliably used or not, and whether energy
efficiency practices succeed in limiting the growth in energy
Swiss and Austrian team concluded that “improving the
efficiency of energy” is key, because
limiting the demand for energy had the largest impact on the
size of the feasibility window. Next in importance was the
availability of CO2 capture and storage techniques,
and the “immediate participation of all regions”
in agreements to cut greenhouse gas emissions. By this they
mean that China, India, and the rest of the developing world
would have to reduce their carbon emissions along with the industrialized First World. Without both changes, the team
added, “it is infeasible to achieve 2°C in our framework.”
Finally, nuclear power does not seem to be absolutely necessary
to meet the 2° target, unless CO2 emissions in
2020 exceed what they are today. (Editor's note: CO2
emissions certainly are on a course to do that.)
challenge to keep global warming below 2°C”
(2013), commentary by G. P. Peters and eight others, Nature
Climate Change, vol. 3, 4-6 (2013). Doi:10.1038/nclimate1783.
Emissions Levels Required to Limit Warming to below 2°C”
(2013), by Joeri Rogelj, D.L. McCollum, B.C O’Neill
and K. Riahi, Nature Climate Change, vol. 3, 405-412,
3. The current amount of greenhouse gases
emitted by human activity was estimated to be equivalent to
51.8 billion tons of CO2 in 2012.
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