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”

“……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 CO 2 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?

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. CO 2 e includes effects of other greenhouses gases, is higher than CO 2. 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 )

What is the climate sensitivity? Note a CO 2 doubling is ~4 W/m^2 so T 2xCO2 is 4 times the x-axis here Paleoclimate (Paleosens Collaboration, Nature )

68% probability 95% probability

Beware the abrupt change Drijfhout et al 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 (non- binding) targets that countries will try to achieve. CO 2 currently around 405 ppm. CO 2 (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 ppm at 95%) – At 50% probability need <450 ppm for 2 degrees ( ppm at 95%) 1 ppm is equivilent to 2.3 Gt of of Carbon (or 7.81 Gt of CO 2 )

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= Year or year projections from climateactiontracker.com Climate sensitivity is the temperature rise for a doubling of CO 2

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 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 INDC pledges are ~55 Gt/yr globally in 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 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 CO 2 ). INDCs (if honoured for a century) will give between 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