[Vision2020] June 2017: Stefan Rahmstorf and Anders Levermann "Why global emissions must peak by 2020"

Ted Moffett starbliss at gmail.com
Mon Jun 19 17:55:03 PDT 2017

Who realistically believes global human sourced atmospheric CO2 emissions
will peak and drop precipitously in the time frame this article indicates
must happen to avoid catastrophic global warming?
Note chart indicating largely irreversible tipping points, and mention that
WAIS (West Antarctic ice sheet) has already likely been "...destabilised,
committing the world to at least three meters of global sea-level rise in
coming centuries..."
Vision2020 Post: Ted Moffett
why-global-emissions-must-peak-by-2020/#_edn3Why global emissions must peak
by 2020
Filed under:

   - Climate Science
   - Greenhouse gases
   - Solutions

— stefan @ 2 June 2017

*(by Stefan Rahmstorf and Anders Levermann)*

In the landmark Paris Climate Agreement, the world’s nations have committed
to “holding the increase in the global average temperature to well below 2
°C above pre-industrial levels and to pursue efforts to limit the
temperature increase to 1.5 °C above pre-industrial levels”. This goal is
deemed necessary to avoid incalculable risks to humanity, and it is
feasible – but realistically only if global emissions peak by the year 2020
at the latest.

Let us first address the importance of remaining well below 2°C of global
warming, and as close to 1.5°C as possible. The World Meteorological
Organization climate report[i]
for the past year has highlighted that global temperature and sea levels
keep rising, reaching record highs once again in 2016. Global sea ice cover
reached a record low, and mountain glaciers and the huge ice sheets in
Greenland and Antarctica are on a trajectory of accelerating mass loss.
More and more people are suffering from increasing and often unprecedented
extreme weather events[ii]
both in terms of casualties and financial losses. This is the situation
after about 1°C global warming since the late 19th Century.

Not only will these impacts get progressively worse as warming continues,
but our planet also runs a growing risk of crossing critical tipping points
where major and largely irreversible changes to the Earthsystem are
triggered (see Fig. 1).

*Fig. 1** Tipping elements in the Earth system, in relation to past global
temperature evolution since the last Ice Age 20,000 years ago as well as
future warming scenarios[iii]
The Paris range of 1.5 – 2 °C warming is shown in grey; the bars show
increasing risk of crossing tipping points from yellow to red.*

The West Antarctic Ice Sheet (WAIS in Fig. 1) has likely already been
destabilised, committing the world to at least three meters of global
sea-level rise in coming centuries[iv]
– an outcome that scientists have warned about since the 1970s[v]
The Greenland Ice Sheet – holding enough ice to eventually raise global sea
levels by seven meters – may likewise be destabilised below 2°C[vi]
Coral reefs have suffered pan-tropical mass bleaching in 2016 and are doing
so again in 2017 as a result of warming oceans, and only if global
temperature stays well below 2°C some remnants of the world’s coral reefs
can be saved[vii]
The Gulf Stream system (THC in Fig. 1) appears to be already slowing[viii]
and recent research indicates it is far more unstable than previously

Because overall global temperature rise depends on cumulative global CO2
emissions, the Paris temperature range can be translated, with some
uncertainty, into a budget of CO2emissions that are still permissible. This
is the overall budget for the century and it lies within the range of 150
to 1050 Gt of CO2, based on updated numbers from IPCC[ix]
At the current global emission level of 39 GtCO2 per year, the lower limit
of this range would be crossed in less than four years and is thus already
unachievable without massive application of largely unproven and
speculative carbon dioxide removal technologies. Even the CO2 budget
corresponding to the mid-point of this uncertainty range, 600 GtCO2, is
equivalent to only 15 years of current emissions. Fig. 2 illustrates three
scenarios with this budget and different peaking years for global
emissions. It makes clear that even if we peak in 2020 reducing emissions
to zero within twenty years will be required. By assuming a more optimistic
budget of 800 Gt this can be stretched to thirty years, but at a
significant risk of exceeding 2°C warming.

It is still possible therefore to meet the Paris temperature goals if
emissions peak by 2020 at the latest, and there are signs to show we are
moving in that direction as global CO2 emissions have not increased for the
past three years. We will need an enormous amount of action and scaled up
ambition to harness the current momentum in order to travel down the
decarbonisation curve at the necessary pace; the window to do that is still

In summary, declining carbon emissions after 2020 is a necessity for
meeting the Paris temperature limit of “well below 2 degrees”.

*Fig. 2** Three illustrative scenarios for spending the same budget of 600
Gt CO2, with emissions peaking in 2016 (green), 2020 (blue) and 2025 (red),
and an alternative with 800 Gt (dashed).*

*Note: *This article first appeared in the report 2020 The Climate Turning


World Meteorological Organisation. WMO Statement on the State of the Global
Climate in 2016 (WMO, Geneva, 2017).

World Meteorological Organisation. Weather extremes in a changing climate:
hindsight on foresight (WMO, Geneva, 2011).

Schellnhuber, H. J., Rahmstorf, S. & Winkelmann, R. Why the right climate
target was agreed in Paris. Nature Climate Change 6, 649-653 (2016).

Feldmann, J. & Levermann, A. Collapse of the West Antarctic Ice Sheet after
local destabilization of the Amundsen Basin. Proc Natl Acad Sci U S A 112,
14191-6 (2015). doi:10.1073/pnas.1512482112

Mercer, J. West Antarctic ice sheet and CO2 greenhouse effect: a threat of
disaster. Nature 271, 321-325 (1978).

Robinson, A., Calov, R. & Ganopolski, A. Multistability and critical
thresholds of the Greenland ice sheet. Nature Climate Change 2, 429-432
(2012). doi:10.1038/nclimate1449

Frieler, K. et al. Limiting global warming to 2 degrees C is unlikely to
save most coral reefs. Nature Climate Change 3, 165-170 (2013). doi:Doi

Rahmstorf, S. et al. Exceptional twentieth-century slowdown in Atlantic
Ocean overturning circulation. Nature Climate Change 5, 475-480 (2015).

Peters, G. How much carbon dioxide can we emit?  (2017)

A Roadmap for Rapid Decarbonization. Science, March 24, 2017: Johan
Rockström, Owen Gaffney, Joeri Rogelj, Malte Meinshausen, Neboja
Nakicenovic, Hans Joachim Schellnhuber http://www.
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