Estimates of global warming from the late 19th Century, from Berkeley Earth, GISTEMP, HadCRUT4, Cowtan and Way, and NCEI. Graphic: Gavin Schmidt / Twitter

By Dana Nuccitelli
27 September 2017

(The Guardian) – Last week, Nature Geoscience published a study suggesting that we have a bigger remaining carbon budget than previously thought to keep global warming below the 1.5°C aggressive Paris climate target. Many scientists quickly commented that the paper’s conclusion was based on some questionable assumptions, and this single study shouldn’t be blindly accepted as gospel truth.

Conservative media outlets did even worse than that. They took one part of the paper’s analysis out of context and grossly distorted its conclusions to advance their anti-climate agenda.

1.5°C might indeed be a geophysical impossibility

The study used the UK Met Office and Hadley Centre’s HadCRUT4 global temperature data set to conclude that so far we’ve warmed 0.93°C from the mid-1800s to 2015, compared to the Paris target of 1.5°C above pre-industrial temperatures. Several climate scientists immediately noted a problem here – HadCRUT4 excludes the Arctic region, which is the fastest-warming part of the planet. Hence it’s one of the least globally-representative temperature datasets. According to more globally-complete data sets like Berkeley Earth, the warming we’ve seen is closer to 1.1°C.

Defining “pre-industrial temperatures” is another issue. Humans caused some global warming prior to the mid-1800s; as one recent study showed, as much as 0.2°C.

A third problem discussed by climate scientists Stefan Rahmstorf at RealClimate involves the way the study authors defined the budget itself. They looked at how much carbon will be emitted at the time we reach 1.5°C warming, but because of what’s known as the ‘thermal inertia’ of the oceans, and because sunlight-reflecting pollutants will fall out of the atmosphere as we shift away from dirty coal power, the planet will keep warming after that time.

If we take all these factors together, depending on how we decide to define “pre-industrial” in the Paris target, we may in fact already be committed to 1.5°C warming, and the headline conclusion that “the 1.5C warming limit is not yet a geophysical impossibility” may be incorrect.

But ultimately that’s a relatively unimportant point. Even if limiting global warming to 1.5°C is still technically feasible, it will take immense global action to achieve it. If it’s infeasible, we still need immense global action to try and stay below 2°C, or 2.5°C, or to slow global warming as much as possible to avoid catastrophic consequences.

From a real-world policy perspective, we need all hands on deck whether the conclusions of this paper are right or wrong. [more]

Right-wing media could not be more wrong about the 1.5°C carbon budget paper


ABSTRACT: The Paris Agreement has opened debate on whether limiting warming to 1.5 °C is compatible with current emission pledges and warming of about 0.9 °C from the mid-nineteenth century to the present decade. We show that limiting cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 warming to less than 0.6 °C in 66% of Earth system model members of the CMIP5 ensemble with no mitigation of other climate drivers, increasing to 240 GtC with ambitious non-CO2 mitigation. We combine a simple climate–carbon-cycle model with estimated ranges for key climate system properties from the IPCC Fifth Assessment Report. Assuming emissions peak and decline to below current levels by 2030, and continue thereafter on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0 °C above the mid-nineteenth century. If CO2 emissions are continuously adjusted over time to limit 2100 warming to 1.5 °C, with ambitious non-CO2 mitigation, net future cumulative CO2 emissions are unlikely to prove less than 250 GtC and unlikely greater than 540 GtC. Hence, limiting warming to 1.5 °C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction rates proving economically, technically or politically unfeasible.

Emission budgets and pathways consistent with limiting warming to 1.5 °C

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