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Optimistic climate paper

David Klein

A new study, not without controversy, has been published that asserts that 1.5C of warming is still possible. The paper is posted here:

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

A popular version including disagreements are posted here:

Science & Environment

Paris climate aim 'still achievable'
By Paul RinconScience editor, BBC News website

Here is a BBC report that includes criticism of the study:



Brad H

Can anyone please post the most authoritative article that says the opposite - that staying under 1.5C is officially NOT possible? Thanks!

David Klein

Well, this new paper seems to go against the IPCC consensus that RCP 2.6 (essentially staying below 2C) requires that we reach zero emissions well before the end of the century, and with most such studies finding that substantial negative emissions on top of that would be required.


Ted F

Here is how the discussion evolved in an email chain reacting to David Klein's original post. We have moved the email chain here to the forum so that more people can benefit from a public exchange on this subject.

On Tuesday, September 19, 2017 at 2:38 PM, Phil Gasper wrote
Sure, it's physically possible [to stay under 1.5C]. But as Michael Mann says in the article, it would require ending carbon emissions immediately. Even if Mann is mistaken, it would require emission cuts much more drastic than anything currently proposed, beginning right now. I wish there was a political movement powerful enough to bring that about, but unfortunately that's not yet the case by a very long shot.
On Tuesday, September 19, 2017 at 2:40 PM, Steve Ongerth wrote
The article does *not* say anything about “ending carbon emissions immediately”; it says we must reduce them by 4-6% per year; no easy task, but somewhat different than “ending carbon emissions immediately”.
On Tuesday, September 19, 2017 at 3:08 PM, Phil Gasper wrote
I guess one of us should read more carefully:

Prof Michael Mann, said the latest research in Nature Geoscience, "doesn't account for [the] pre-industrial baseline issue we examined".

He added: "There is some debate about [the] precise amount of committed warming if we cease emitting carbon immediately. We're probably very close to 1.5C."
On Tuesday, September 19, 2017 at 3:19 PM, Steve Ongerth wrote
I was referring to the study itself, not Prof Mann’s opinion *on* the study. His argument about the pre-industrial levels may be true, but that’s not been conclusively proven, yet.
On Tuesday, September 19, 2017 at 5:44 PM, Dan wrote
Actually, I think 1.5◦ C is not ambitious enough. I haven't read this new study, but apparently it calls for emission cuts of 4-6% a year? I've called in print for a much faster phasedown than that.

I say we should stick to the 1◦C target demanded by social movements in Cochabama in 2010. In a study last year, Hansen wrote that stabilization within a century at 1◦C is still possible:

"The 350 ppm CO2 target is moderately stricter than the 1.5◦ C warming target. The near-planetary energy balance anticipated at 350ppm CO2 implies a global temperature close to recent values, i.e., about +1◦ C relative to preindustrial. We advocate pursuit of this goal within a century." (page 1)


The study says that "improved agricultural and forestry practices, including reforestation and steps to improve soil fertility can increase its carbon content, may provide much of the necessary CO2 extraction."

While Hansen's study also considers an amount of geoengineering to be necessary, I'm not convinced. Couldn't the sorts of "politically impossible" social transformations advocated by us ecosocialists sufficiently speed up the necessary economic changes without resorting to geoengineering? Even if it means shutting down the 90 companies responsible for 2/3 of emissions, even if it means the Global North embracing buen vivir attitudes about consumption, even if it means embracing "revolution" with all the associated uncertainties, etc.
On Wednesday, September 20, 2017 at 8:42 AM, David Klein wrote
>>While Hansen's study also considers an amount of geoengineering to be necessary, I'm not convinced. Couldn't the sorts of "politically impossible" social transformations advocated by us ecosocialists sufficiently speed up the necessary economic changes without resorting to geoengineering?<<

The short answer is no. To explain why, consider that from 1750 to 2011 is 555 billion tons were emitted by humanity according to the IPCC, of which 375 billion tons came from burning fossil fuels and 180 billion tons from deforestation. That means that the upper bound for absorption of CO2 through afforestation and reforrestation is 180 billion tons and not an ounce more, but it is hard to imagine the vast pre-1750 forrests returning, especially in light of the current human population. There is a basic carbon conservation principle at work here. Hansen et al optimistically estimate a maximum a drawdown by plants and soil of CO2 to be 100 billion tons. And that's really pushing it.

Plants, even on a massive global scale, can only absorb a fraction of the CO2 humanity has emitted. Keep in mind that a draw down through afforestation and reforrestation is a one shot deal. This is because a mature forrest emits as much CO2 as it aborbs. A net draw down only occurs during initial growth. After that decaying plant life and respiration (during the night) returns as much CO2 as was absorbed through photosynthesis during the day.

Since 2011 much more CO2 has been emitted. For the last two years atmospheric CO2 concentration has gone up by 3 ppm (parts per million) and by at least 2 ppm since 2009 (with one exception). That is a good measure of capitalism's expansion. Each 1 ppm of CO2 is 2.12 billion tons! Plants and soil alone cannot undue all that damage. This is why Hansen et al recognized that CDR (carbon dioxide removal) is a requirement to assure Holocene temperature ranges. But that technology does not even exist at the scale needed, so we're in for some very rough times even if capitalism is crushed.

On Wednesday, September 20, 2017 9:43 AM, Dan wrote

Thanks for your feedback. I'll have to do further research, but for now I wanted to make two points.

First, you say mature forests do not sequester carbon, but this 2016 Nature study says otherwise: "Old-growth forests remove carbon dioxide from the atmosphere...The sequestered carbon dioxide is stored in live woody tissues and slowly decomposing organic matter in litter and soil. Old-growth forests therefore serve as a global carbon dioxide sink."

Second, I think there may be a lot of potential in the "carbon farming" methods described by the permaculturalist Eric Toensmeler. He says that combined with a fossil fuel phasedown, these carbon-sequestering methods of producing food could bring CO2 levels down to 350 ppm:

"In this book the term carbon farming is used to describe a suite of crops and agricultural practices that sequester carbon in the soil and in perennial vegetation like trees. If widely implemented, these practices have the capacity to sequester hundreds of billions of tons of carbon from the atmosphere in the coming decades. And if we combine carbon farming with a massive global reduction in fossil fuel emissions, it can bring us back from the brink of disaster and return our atmosphere to the "magic number" of 350 parts per million of carbon dioxide. Unlike high-tech geoengineering strategies, these practices can also feed people, build more fertile soils, and contribute to ecosystem health."
Excerpt | The Carbon Farming Solution


David Klein

Hi Dan,

It is true that as long as a tree is growing, it is sequestering carbon, and that is true of both older and younger trees. It is also true that when trees die and decay they release their accumulated carbon back to the atmosphere. And when forest fires or insect infestations strike, as they inevitably do, the release of CO2 back to the atmosphere is massive and rapid.

The 2008 Nature study letter that you referenced did not mention forest fires, but the abstract to the research paper said this:
"...our findings suggest that 15 per cent of the global forest area, which is currently not considered when offsetting increasing atmospheric carbon dioxide concentrations, provides at least 10 per cent of the global net." [bold added]​

The Nature editor's summary of the paper drove this point home:

"The findings suggest that old-growth forests can continue to accumulate carbon, and that they contribute at least 10% of global net ecosystem productivity. Much of this carbon, even soil carbon, will move back to the atmosphere if these forests are disturbed, so it would make sense for carbon accounting rules to give credit for leaving old forests intact."[bold added]​

Unfortunately corporate offsets and profit --- typically part of cap-and-trade schemes --- motivates this line of research, so caution in embracing it is in order. But there is also more recent credible research in the opposite direction. For example this 2017 paper:

Net CO2 emissions from a primary boreo-nemoral forest over a 10 year period
by David Hadden, Achim Grelle, in: Forest Ecology and Management, Volume 398, 15 August 2017, Pages 164-173

Quoting from the abstract:

"Primary forests play an important role in the global carbon balance. With little to no human intervention, primary forests are shaped and characterised by disturbances such as weather extremes, fire, insect and pathogen attacks. Such disturbances have a direct impact on the volume of coarse woody debris (CWD) which contributes to the total ecosystem respiration (Re). There are currently few studies that present continuous long term measurements of the carbon balance of northern primary forests. We used the eddy covariance method to measure continuous carbon dioxide (CO2) fluxes from a Swedish primary boreo-nemoral forest over a ten year period. By mapping the measured CO2 fluxes to the forest ecosystem we could indicate that small areas that had some form of disturbance and areas with significant levels of CWD within the eddy covariance footprint contributed to the total Re resulting in the forest being a net carbon source." [bold added]

Think about Earth as a whole. There is no reason to believe that humans can sequester more (or even as much) carbon in forests and grasslands as nature was able to do for millions of years before the anthropocene. So adding 180 billion (metric) tons back into the soil, plants, and trees is the maximum possible without geoengineering. That is the amount (as of 2011) that has been released to the atmosphere through massive deforestation. This is an upper bound for the planet with virtually all land area on the planet reforested.

Capitalism has not only destroyed forests but has also reached back in time to the Carboniferous period when vast deposits of coal were produced as fluctuating seas drowned the tropical forests that covered North America and Europe. So in a sense, capitalism has been burning both existing trees and the remnants of trees from that earlier time simultaneously, essentially two planets' worth of carbon --- one from the present and the other from hundreds of millions of years ago. Nature is now overwhelmed and does not have the capacity to sequester two planet's worth of carbon emissions on any time scale relevant to humans.

Hansen and his co-authors did the calculations and research to take all of these carbon budgets into account. Afforestation and reforestation are valuable and important, but by themselves do not constitute a panacea for drawing down atmospheric CO2 concentrations to Holocene levels.


Sandra Lindberg

Hi Dan,

"It is true that as long as a tree is growing, it is sequestering carbon, and that is true of both older and younger trees. It is also true that when trees die and decay they release their accumulated carbon back to the atmosphere. And when forest fires or insect infestations strike, as they inevitably do, the release of CO2 back to the atmosphere is massive and rapid..."

Here's the reply I posted to the listserve:

This thread coincides with environmental activism happening in Decatur IL right now. The City has been approached by an unscrupulous tree removal company that is offering to pay the City $25,000 for 114 mature, hardwood trees (oak and walnut). A consortium of local activists called the Community Environmental Coalition is fighting City Hall on this one. So far we've kept the chainsaws away.

This trend--municipalities cutting down mature trees for small amounts of profit--seems to be troubling Europe as well as the US. The Guardian had an article about the need to preserve London's lime and apple trees in the face of city government all too eager to cut them down: "Introducing treeconomics: how street trees can save our cities" Introducing 'treeconomics': how street trees can save our cities.

The Guardian article taught me about a USDA suite of programs called iTree that allows a city to measure the 'value' of its entire tree canopy, on both municipal, county and private land: i-Tree. This site talks about the need to think much more extensively about the value of trees. In Decatur, I'm going to bring this software to the attention of city government. The software is free and is apparently being used in Europe in quite a few places.

Neoliberal capitalist ideas are in part to blame for this trend that is bringing so-called harvesting to trees on the commons. As the right convinces municipalities to cut service aspects of city budgets, including tree removal and tree maintenance budgets, for-profit businesses show up at city hall offering to lighten the city's responsibility for trees by cutting them down. It's a particularly rotten development as this trend contributes to increased CO2 levels and climate change in multiple ways.

On the truly micro level--and I offer this so that people who are responsible for trees will think about this possibility--the permaculture notion of finding a way to keep any organic matter generated on a particular piece of property on that property offers ideas for handling dead and/or diseased trees so that they lessen CO2 aspects of their existence. We have two dead pines, a young Hackberry too close to a fence and dead branches in an oak and maple. Our arborist is cutting down this wood and leaving the trunks and large branches in 8' lengths that we are using to edge new vegetable beds we are putting in. Some of this wood may be buried to make berms that will keep rainwater on our property and lessen run-off. Every bit of wood we use in this way means lumber we will not purchase. Leaving the 8' trunks whole will greatly slow down how quickly they break down and release their CO2 into the environment. If every piece of land treated necessary removal of dead or dying wood in this way, communities could significantly lessen wood purchases, could decrease CO2 release far more than would happen if the trees were treated conventionally (cut into small sections and chipped), and would lessen dead wood going into a landfill rather than being repurposed.

I completely agree with David that practices such as these, as well as organic farming that increases the soil's ability to sequester CO2 far better than industrial ag fields can, cannot alone significantly impact CO2 levels, but I want to emphasize that every time we individually and collectively adopt approaches that carefully examine every tree before it comes down, and how that downed tree will be treated after it hits the ground, we become part of small but many global efforts to do our part in addressing the calamity before us. Our individual and small group actions like this, just like the thousands demonstrating against business as usual around the globe, contribute to global, collective pressure that we also need in order to shift government and business practices in the short term--before the revolution gets here.

It ain't over until it's over,


Dan Fischer

New Member

I'm looking at Hansen's math. He says 153 billion tons of 'negative CO2 emissions' would be required with a 6% annual reduction of greenhouse gas emissions. Presumably, a faster emission reduction would reduce the required negative emissions to a lower amount, such as 100 billion tons. Hansen says the Earth can sequester 100 billion tons through improved forestry and agriculture.

Couldn't an ecosocialist society cut emissions by 6% or more annually and radically improve forestry and agriculture? Imagine a worldwide revolution happens really soon. Consumption levels drop rapidly and the workweek shrinks to a fraction of today's. Neighborhoods get power from locally-controlled renewables, but there isn't a smokestack in sight. There are hardly any cars, let alone tanks. Gardens and forests get planted practically everywhere.

In this scenario, what's the best we could hope for? I don't mind 'demanding the impossible' as long as it's not physically impossible. So my question is: What are the most stringent temperature and GHG stabilization targets that can still be physically met?


David Klein

Hi Dan,

You asked,

"In this scenario, what's the best we could hope for? I don't mind 'demanding the impossible' as long as it's not physically impossible. So my question is: What are the most stringent temperature and GHG stabilization targets that can still be physically met?"

I suppose that the best possible scenario (as a limiting case) would be an immediate cessation of all GHG emissions, so zero emissions starting today, together with a massive immediate afforestation/reforestation project adopted worldwide.

The IPCC addressed the zero-emissions scenario (but without an accompanying massive afforestation/reforestation project) in AR5 WG1 FAQ 12.3. This is excerpted from the report:

FAQ 12.3 | What Would Happen to Future Climate if We Stopped Emissions Today?
Even if anthropogenic greenhouses gas emissions were halted now, the radiative forcing due to these long- lived greenhouse gases concentrations would only slowly decrease in the future, at a rate determined by the lifetime of the gas (see above). Moreover, the climate response of the Earth System to that radiative forcing would be even slower. Global temperature would not respond quickly to the greenhouse gas concentration changes. Eliminating CO2 emissions only would lead to near constant temperature for many centuries. Eliminating short-lived negative forcings from sulphate aerosols at the same time (e.g., by air pollution reduction measures) would cause a temporary warming of a few tenths of a degree, as shown in blue in FAQ 12.3, Figure 1. Setting all emissions to zero would therefore, after a short warming, lead to a near stabilization of the climate for multiple centuries. This is called the commitment from past emissions (or zero future emission commitment). The concentration of GHG would decrease and hence the radiative forcing as well, but the inertia of the climate system would delay the temperature response.​

Here is a specific study on this question from 2014:

Continued global warming after CO2 emissions stoppage
Thomas Lukas Frölicher, Michael Winton & Jorge Louis Sarmiento
Nature Climate Change 4, 40–44 (2014)

Here is the finding:

"After a century of cooling, the planet warmed by 0.37 degrees Celsius (0.66 Fahrenheit) during the next 400 years as the ocean absorbed less and less heat."​

In other words, if the world stopped all GHG emissions today, the global average temperature would be about what it is now for a long time.

This does not take into account an accompanying massive global afforestation/reforestation project. I'm not sure how to quantify the effects of such a hypothetical project. The rate of draw down of CO2 would be the same as the rate of growth of forests and grasslands worldwide. Hansen et al estimate a maximum draw down of 100 billion tons of Carbon from this process eventually. That would eventually reduce the global average temperature to below 1C of warming above pre-industrial times. All of this is of course extremely hypothetical and serves only as a limiting case for what is possible.


Ted F

I'm confused, David. The AR5 WG1 FAQ 12.3 says
The climate system response to the greenhouse gases and aerosols forcing is characterized by an inertia, driven mainly by the ocean. The ocean has a very large capacity of absorbing heat and a slow mixing between the surface and the deep ocean. This means that it will take several centuries for the whole ocean to warm up and to reach equilibrium with the altered radiative forcing. The surface ocean (and hence the continents) will continue to warm until it reaches a surface temperature in equilibrium with this new radiative forcing. The AR4 showed that if concentration of greenhouse gases were held constant at present day level, the Earth surface would still continue to warm by about 0.6°C over the 21st century relative to the year 2000.
But you say
In other words, if the world stopped all GHG emissions today, the global average temperature would be about what it is now for a long time.
Are you saying 0.6°C of warming is keeping the global average temperature "about what it is now"? Or are you saying that stopping all GHG emissions today is different from holding the concentration of greenhouse gases constant at present day level?

Ted F

This is an interesting caveat in the AR5 WG1 FAQ 12.3
Global temperature is a useful aggregate number to describe the magnitude of climate change, but not all changes will scale linearly global temperature. Changes in the water cycle for example also depend on the type of forcing (e.g., greenhouse gases, aerosols, land use change), slower components of the Earth system such as sea level rise and ice sheet would take much longer to respond, and there may be critical thresholds or abrupt or irreversible changes in the climate system.
Not that we need any further warnings, but it appears the best case scenarios we are discussing assume that we haven't already crossed any critical thresholds or provoked as yet unseen abrupt or irreversible changes.

David Klein

It's not even a best case scenario because the hypotheticals --- dropping to zero emissions immediately and instituting a massive global reforestation/afforestation program --- won't happen in the real world. This only represents a physical limit well beyond what can be realistically achieved and is an absolute theoretical barrier to any possible progress we can achieve. There are a lot of scientific studies that look at limiting cases of what is possible. You shouldn't consider this to be a realistic possible outcome.


David Klein

I'm confused, David. The AR5 WG1 FAQ 12.3 says

But you say

Are you saying 0.6°C of warming is keeping the global average temperature "about what it is now"? Or are you saying that stopping all GHG emissions today is different from holding the concentration of greenhouse gases constant at present day level?
I'm saying that stopping all GHG emissions today is different from holding the concentration of greehnouse gases constant at present day levels. Here is a little background:

"Some greenhouse gases and aerosols are retained for days to years in the atmosphere after emission. The concentrations of such compounds in the atmosphere are tightly coupled to the rate of emission. Their concentrations would drop rapidly if emissions were to cease. Increasing emissions lead to increases in concentrations of such gases, while constant emissions are required for their concentrations to be stabilized."​

The above passage and more details appear here:



David Klein

More controversy about this research paper. What do you think?


Did we just buy decades more time to hit climate goals?
Akshat Rathi

September 21, 2017
When the Paris climate accord was signed, environmentalists were relieved that finally the world had come together and agreed to work on mitigating climate change. At the same time, many experts were worried about achieving the goals that were set: to keep global warming well below 2°C of pre-industrial levels and try to keep it under 1.5°C.

Though it seemed possible to keep warming under 2°C, many experts believed there was no way we won’t exceed 1.5°C. That’s because we’ve already crossed the 1°C threshold, and many argue that even if the world stopped emitting all carbon dioxide today, the globe will continue to heat up. Carbon dioxide is highly effective at storing the sun’s energy, and some calculations showed that continued accumulation of the heat in the atmosphere would be enough to go beyond the 1.5°C mark.

To simplify the difficulty of understanding what we must do to hit these goals, the world’s leading experts worked with the Intergovernmental Panel on Climate Change (IPCC) to the introduce the idea of a “carbon budget.” It estimates how much carbon dioxide the world can still emit while still having a likelihood of keeping global warming under a certain threshold.

As of April 2017, the IPCC’s carbon budget for keeping temperature rise below 1.5°C was approximately 160 billion metric tonnes of carbon dioxide. At the current annual rate of emissions—about 40 billion metric tonnes—it would mean the budget would be exhausted in the next four years.

A clarification from the world’s leading experts
Now a new study, published in Nature Geoscience, seems to have changed the game. It suggests that the 1.5°C carbon budget may be closer to about 800 billion metric tonnes. That would buy the world as much as 20 years at current emission levels. The reason to take the study seriously is that it’s published by the world’s leading experts on carbon budgets.

The study, however, seems to have been interpreted poorly in some media reports. That’s forced the lead author Richard Millar of the University of Oxford and his colleagues to publish a clarification:

A number of media reports have asserted that our recent study in Nature Geoscience indicates that global temperatures are not rising as fast as predicted by the IPCC, and hence that action to reduce greenhouse gas emissions is no longer urgent. Both assertions are false.

Why the confusion?
The reason for the confusion has to do with the uncertainties associated with calculating carbon budgets. Glen Peters of Center for International Climate Research finds that, depending on what factors are included, the 1.5°C carbon budget could be anywhere between –200 billion metric tonnes to +1,000 billion metric tonnes—that is, either we’ve already blown past the budget or we have lots of leeway. This measure of uncertainty is not good for policy making.

“The carbon budget concept is a brilliant way to illustrate the importance of zero emissions, the need for rapid mitigation, and to compare different temperature targets,” Peters writes. “The carbon budget concept is less useful for emission pathways, particularly with tight carbon budgets and given that it is possible to exceed the carbon budget using negative emissions.”

The clear answer
So what do policymakers take away from the latest study? Millar and colleagues are clear: “To likely meet the Paris goal, emission reductions would need to begin immediately and reach zero in less than 40 years.”

In short, the urgency with which we must reduce emissions and reach net-zero emissions hasn’t changed.

Nature | News

Limiting global warming to 1.5 °C may still be possible
Analysis suggests that researchers have underestimated how much carbon humanity can emit before reaching this level of warming.

18 September 2017

Coal-fired power plants, such as this one in Germany, are a source of greenhouse-gas emissions.

A team of climate scientists has delivered a rare bit of good news: it could be easier than previously thought to limit global warming to 1.5 °C above pre-industrial levels, as called for in the 2015 Paris climate agreement. But even if the team is right — and some researchers are already questioning the conclusions — heroic efforts to curb greenhouse-gas emissions will still be necessary to limit warming.

Published on 18 September in Nature Geoscience1, the analysis focuses in part on the fact that global climate models used in the 2013 report from the Intergovernmental Panel on Climate Change (IPCC) tend to overestimate the extent of warming that has already occurred. After adjusting for that discrepancy and running further models, the authors of the latest study found that the amount of carbon that humanity can emit from 2015 onward while holding temperatures below 1.5 °C is nearly three times greater than estimated by the IPCC — or even larger if there is aggressive action on greenhouse gases beyond carbon dioxide.

The implications for global policymakers are significant. Humanity is poised to blow through the IPCC’s carbon budget for a 1.5 °C rise within a few years, leading many scientists to declare the goal impossible. But the new analysis suggests that it could be met with a modest strengthening of the current Paris pledges up to 2030, followed by sharp cuts in carbon emissions thereafter.

“The Paris goal of 1.5 °C is not impossible — it’s just very, very difficult,” says lead author Richard Millar, a climate researcher at the University of Oxford, UK.

Debate rages on
The work is receiving mixed reviews. Some argue that the analysis is fundamentally flawed, because it centres on a period of slower warming that began around the turn of the millennium. This period, often referred to as the climate hiatus, continued until 2014. Scientists think that natural variability in the climate system temporarily suppressed temperatures during this period.

The team’s estimate for the amount of warming that humans have caused so far — 0.93 °C — could thus be artificially low, because it calculates the human contribution to warming during this cooler time, says Ben Sanderson, a climate modeller at the National Center for Atmospheric Research in Boulder, Colorado.

At the same time, he says, the oceans and the land were probably absorbing more carbon than normal during this period. Natural processes will eventually dump some of that back into the atmosphere, thus reducing the amount of carbon that humanity can emit before reaching 1.5°C.

“These two effects, to my mind, explain away their result and reinforce the original IPCC conclusion,” Sanderson says.

But Millar and his colleagues argue that the effects of the hiatus would be minimal. The team used multiple methodologies to estimate the actual warming due to greenhouse gases, independent of short-term climate variability. The scientists calculated how much carbon would be needed to push the temperature up by another 0.6 °C, to 1.5 °C. But they also calculated how much carbon it would take to reach that threshold if the amount of human-caused warming so far was lower or higher than their estimate of 0.93 °C.

In all cases, Millar says, the amount of carbon that humans could emit before Earth warms to that 1.5 °C threshold is larger than previously estimated.

Counting carbon
Nathan Gillett, a climatologist at the Canadian Centre for Climate Modelling and Analysis in Victoria, says that other teams have previously documented the slight discrepancy between the warming projected by climate models and that shown by actual observations. But Gillett credits Millar’s team with teasing out the implications of this gap, and of reducing the uncertainty surrounding the amount of emissions that would produce warming of 1.5 °C. “I think their central conclusion is robust,” Gillett says.

The debate over how close the world is to the 1.5 °C warming threshold is unlikely to be resolved any time soon, but one thing is clear: modelling scenarios that enable Earth to remain below that target poses a new kind of challenge. Uncertainty about the details of humanity’s carbon budget don’t matter so much when scientists are modelling the cumulative effect of greenhouse gases over the course of centuries. But fine details matter a great deal when researchers are looking at what level of greenhouse-gas emissions would bump warming to 1.5 °C, because, in that case, scientists’ goal is to tease out the precise effects of heat-trapping gases over a few decades.

“When we start thinking about really ambitious mitigation goals in the really near term, everything starts to matter,” Millar says.

That is true for science as well as for climate policy. “For a lot of people, it would probably be easier if the Paris goal was actually impossible,” Millar says. “We’re showing that it’s still possible. But the real question is whether we can create the policy action that would actually be required to realize these scenarios.”

[paste:font size="6"]http://dx.doi.org/10.1038/NGEO3031 (2017).