In a reversal, climate scientists now say goal set in Paris accord is within reach

With hopeful news on global warming being such a rarity, it’s worth a few minutes to consider one of the more prominent recent retreats from total gloom.

In an analysis published Friday in the respected journal Nature Geoscience, a team of chiefly British scholars concludes that the Paris Agreement’s goal of holding the line on Earth’s rising temperature to 1.5 degrees Celsius in fact possible.

Possible … but challenging. Challenging … but maybe not as excruciating as it looked at the time the global goal and national targets were established in December 2015.

Back then, the climate economist Michael Grubb of University College London was widely quoted as saying the only conceivable pathways to success would require cutting carbon dioxide emissions at a pace “incompatible with democracy.”

As it happens, the same Michael Grubb is an author of the new paper, which finds that fresher data on the pace of CO2 emissions reduction and adoption of non-fossil-fuel energy systems — along with a significant recalculation of how much more CO2 the atmosphere can handle before going over 1.5°C — put the Paris goal within reach.

As you might expect, the deniers are having a field day with this shift in outlook, as they do with anything susceptible to being portrayed as error by those know-it-all climate scientists.

As for Grubb, he told The Times of London:

When the facts change, I change my mind, as [the economist John Maynard] Keynes said. It’s still likely to be very difficult to achieve these kind of changes quickly enough but we are in a better place than I thought.

The key fact that has changed in the view of Grubb and fellow authors is the pace of warming since the beginning of this millennium, which they now find to be somewhat slower than the forecast used in the Paris talks, and derived from a suite of used models by the Intergovernmental Panel on Climate Change.

This reflects a variety of factors, the largest including faster-than-expected adoption of clean energy alternatives and larger-than-expected reductions in the growth of new emissions in in some large industrial economies, especially China’s. Smaller factors also played a role, including a longer-than-expected persistence of reflective atmospheric particles that block incoming solar radiation.

Expanding the ‘carbon budget’

Taken together, the adjustments mean that the “carbon budget” — how much more carbon we can load into the atmosphere without pushing the temperature increase about 1.5°C — is far greater than the 70-gigaton figure used in the Paris discussions, which would have been used up in five years or less. Now it’s looking more like 240 gigatons.

If the planet stays within that budget, and continues on its current path of controlling methane and other globe-warming gases, the team calculates a two-in-three chance of staying under 1.5°C above pre-industrial levels, or about another 0.6°C above today’s temperatures.

From Oxford University’s announcement of the findings:

Three approaches were used to evaluate the outstanding ‘carbon budget’ (the total amount of CO2 emissions compatible with a given global average warming) for 1.5°C: re-assessing the evidence provided by complex Earth System Models, new experiments with an intermediate-complexity model, and evaluating the implications of current ranges of uncertainty in climate system properties using a simple model. In all cases the level of emissions and warming to date were taken into account.

Dr. Richard Millar, lead author and post-doctoral research fellow at the Oxford Martin Net Zero Carbon Investment Initiative at Oxford University, said: “Limiting total CO2 emissions from the start of 2015 to beneath 240 billion tons of carbon (880 billion tons of CO2), or about 20 years’ of current emissions, would likely achieve the Paris goal of limiting warming to 1.5°C above pre-industrial levels.”

“Previous estimates of the remaining 1.5°C carbon budget based on the IPCC 5th Assessment were around four times lower, so this is very good news for the achievability of the Paris targets,” notes Professor Pierre Friedlingstein of the University of Exeter, a co-author on this study and a key expert on carbon budgets for the Intergovernmental Panel on Climate Change (IPCC).

Achievable does not mean simple or easy, of course. The Nature Geoscience paper says that “limiting warming to 1.5°C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges [under the Paris accord] for 2030 followed by challengingly deep and rapid mitigation.”

After noting that emissions reductions along the lines deemed necessary by the IPCC have only occurred in circumstances like the America’s depression of the 1930s and the collapse of the Soviet Union, the paper says,

More ambitious near-term mitigation may be more feasible than previously thought. The rapid growth of global emissions from 2000 to 2013 was dominated by increases in Chinese emissions, driven, at least in part, by unprecedented levels of debt-fuelled investment in carbon-intensive industries and capital stock. Sustaining such expansion is likely to be neither necessary (the infrastructure is now built) nor feasible (the debt levels are likely to prove unsustainable).

For these reasons, the possibility that both Chinese and global emissions are at or near their peak, and could reduce from 2020, seems less far-fetched than it once did. This could allow for the required strengthening of the [national goals laid out in Paris] … more readily consistent with a 1.5°C goal.

Experts endorse the shift

Expert reaction to the study appears to be generally favorable in assessing the methods and supportive in endorsing the authors’ cautions against irrationally exuberant optimism. A sampling:

David Shukman, science editor for BBC: “The [climate] models are simulated approximations of possible futures. Inevitably they are going to be at least slightly adrift of reality, either in the amount of warming or its timing….  In many ways, it’s remarkable that these computer constructs are even roughly on track. And models designed to come up with very broad potential outcomes for the end of the century may not be fine-tuned enough to give more detailed forecasts year-by-year.

“The authors themselves are anxious that their research is not misunderstood. The need for urgent action to reduce emissions is unchanged, they say. It’s just that the most ambitious of the Paris Agreement targets is not as unachievable as many once thought, that there is time to act, though the task remains a monumental one.”

Ankur Desai, professor in atmospheric and oceanic sciences at University of Wisconsin—Madison, to The Weather Channel: “The study is correct to note that rates of adoption of non-carbon polluting energy sources and fossil fuel emissions reductions are occurring faster than projected in many places. There is a revolution occurring in energy in response to consumer demand and government incentives. However, whether these trends can lead to meeting the more ambitious climate targets of no more than 1.5°C warming is difficult to expect unless nations put teeth into the voluntary emission reduction pledges they all made at Paris…. While not geophysically impossible, it is likely politically implausible, though not outside the reach of possibility.”

Kevin Trenberth, senior scientist at the U.S. National Center for Atmospheric Research, also to the Weather Channel: “Right now, the track we’re headed on, we’ll double carbon dioxide before 2060 which would correspond to 2°C and that’s the problem. I don’t think 1.5°C is achievable, I think 2°C is achievable.”

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The full paper, “Emission budgets and pathways consistent with limiting warming to 1.5°C,” can be found here but access is not free.