Where is the climate heading
after COP21?
Andrew Levan
Physics
“……the increase in the global average temperature to well
below 2 °C above pre- industrial levels and pursuing efforts to
limit the temperature increase to 1.5 °C above pre- industrial
levels”
http://unfccc.int/resource/docs/2015/cop21/eng/10a01.pdf
“……the increase in the global average temperature to well
below 2 °C above pre- industrial levels and pursuing efforts to
limit the temperature increase to 1.5 °C above pre- industrial
levels”
We don’t control the temperature, we control (somewhat) the
equivalent CO2 emissions (which change the concentrations).
So what do we have to do with greenhouse gas concentrations to
meet these targets?
Where are we headed based on what is already promised?
http://unfccc.int/resource/docs/2015/cop21/eng/10a01.pdf
Greenhouse effect primer
Greenhouse effect primer
CO2 emissions lead to a radiative forcing (an imbalance between the energy entering and
leaving the atmosphere)
This gives an accrual of energy in the atmosphere -> temperature rises
Climatic responses are slow. We are not yet in equilibrium.
CO2e includes effects of other greenhouses gases, is higher than CO2. Sometimes also
includes a reduction to allow for negative forcing (particulate matter in the atmosphere,
global dimming).
First principles estimates of temperature rise for a given forcing would only work on a
planet without life, water cycles etc etc, but can still be attempted.
Most attempts to discern outcomes rely on either paleoclimate or modelling.
Climate sensitivity - I
What is the climate sensitivity?
Climate modeling (Roe & Baker Science 2007 319 629)
What is the climate sensitivity?
Paleoclimate (Paleosens Collaboration, Nature 2012 683 691)
Note a CO2 doubling is ~4 W/m^2 so T2xCO2 is 4 times the x-axis here
95% probability
68%
probability
Beware the abrupt change
Drijfhout et al. 2015 PNAS E5777-E5786
Current policy and INDCs
• Countries (EU as a block) submitted their
Intended Nationally Determined Contribution
(INDC) prior to COP21. These are the (nonbinding) targets that countries will try to achieve.
• CO2 currently around 405 ppm.
• CO2(e) around 430 ppm
– Taking into account other greenhouses gases
(470ppm)
– Subtracting for negative forcing (-40 ppm)
What emission paths do we need to
get to 2, or 1.5 degrees?
• Turn the question around:
– What emission levels to we need to reach to have
stay at less than 1.5 or 2 degrees.
– At 50% probability need <400 ppm for 1.5 degrees
(but 325-350 ppm at 95%)
– At 50% probability need <450 ppm for 2 degrees
(350-375ppm at 95%)
1 ppm is equivilent to 2.3 Gt of of Carbon (or 7.81 Gt of CO2)
50% absorbed by oceans
Current policy
Year or year projections from climateactiontracker.com
INDCs
Year or year projections from climateactiontracker.com
2 degrees model
Year or year projections from climateactiontracker.com
1.5 degree model
Year or year projections from climateactiontracker.com
Temperature rise for climate sensitivity=
2.5
3.0
4.5
Current
3.6
4.3
6.5
INDC
2.8
3.4
5.0
2 deg
1.9
2.3
3.4
1.5 deg
1.6
1.9
2.9
Climate sensitivity is the
temperature rise for a
doubling of CO2
Year or year projections from climateactiontracker.com
•
1.5 degree strategies require greenhouse gas emissions to be negative well before 2100!
•
2 degrees would need to half emissions by mid-2030s (this is to the level of the early 1970s), be at
zero by 2080.
•
Rises >2 degrees look likely, unless there is a real international appetite to change emission paths,
and quickly.
Hitting a target
• 1.5 or 2 degree targets require
emissions of <30 Gt/yr by 2030.
• INDC pledges are ~55 Gt/yr
globally in 2030.
• 30 Gt/yr is equal to the INDC
targets of China, India, the EU
and the USA combined.
• Very little wriggle room if any of
these countries fail to do much
better than their targets.
What does it mean?
• A temperature change is just a number, and doesn’t necessarily reflect the
complex series of impacts it can have on e.g. human health, food security,
sea levels, weather patterns, biodiversity etc etc.
• In almost all cases higher temperatures result in larger impacts that are
either costly to deal with, or irreversible.
IPCC AR5
Summary
• COP21 commitments have globally rising emissions throughout
commitment period.
• Assuming efforts accelerate after this point the warming to 2100 should
be in the range 2.5-4.0 degrees, but is more likely to be higher than lower.
• Given current policies (i.e. no new initiatives from INDCs) warming will
probably be >4 degrees.
• It is already almost impossible to hit a 1.5 degree target, and the window
is closing for 2 degrees.
• All of the model predictions to date assume we do not hit any tipping
points. The probability of hitting one increases with the temperature rise.
Skewed climate sensitivity
1 ppm is equivilent to 2.3 Gt of of Carbon (or 7.81 Gt of CO2).
INDCs (if honoured for a century) will give between 3523-4177 Gt CO2)
This is roughly 6ppm/yr. Half probably goes into ocean, but by 2050 we will have CO2
levels of 500pm plus.
2 degree levels require a burning of 2045 Gt, 1.5 degrees 1355 Gt