“Stabilizing climate requires near-zero emissions”

Avoiding climate catastrophe will probably require going to near-zero net emissions of greenhouse gases this century. That is the conclusion of a new paper in Geophysical Research Letters (subs. req’d) co-authored by one of my favorite climate scientists, Ken Caldeira, whose papers always merit attention. Here is the abstract:

Current international climate mitigation efforts aim to stabilize levels of greenhouse gases in the atmosphere. However, human-induced climate warming will continue for many centuries, even after atmospheric CO2 levels are stabilized. In this paper, we assess the CO2 emissions requirements for global temperature stabilization within the next several centuries, using an Earth system model of intermediate complexity. We show first that a single pulse of carbon released into the atmosphere increases globally averaged surface temperature by an amount that remains approximately constant for several centuries, even in the absence of additional emissions. We then show that to hold climate constant at a given global temperature requires near-zero future carbon emissions. Our results suggest that future anthropogenic emissions would need to be eliminated in order to stabilize global-mean temperatures. As a consequence, any future anthropogenic emissions will commit the climate system to warming that is essentially irreversible on centennial timescales.

Since the rest of the article is behind a firewall, let me extract a couple of key findings:

… our results suggest that if emissions were eliminated entirely, radiative forcing from atmospheric CO2 would decrease at a rate closely matched by declining ocean heat uptake, with the result that while future warming commitment may be negligible, atmospheric temperatures may not decrease appreciably for at least 500 years.

In short, the time for dramatic action is upon us. The study concludes:

In the absence of human intervention to actively remove CO2 from the atmosphere, each unit of CO2 emissions must be viewed as leading to quantifiable and essentially permanent climate change on centennial timescales. We emphasize that a stable global climate is not synonymous with stable radiative forcing, but rather requires decreasing greenhouse gas levels in the atmosphere. We have shown here that stable global temperatures within the next several centuries can be achieved if CO2 emissions are reduced to nearly zero. This means that avoiding future human-induced climate warming may require policies that seek not only to decrease CO2 emissions, but to eliminate them entirely.

Bottom line: Stopping global warming is very hard — easily the greatest challenge the human race has ever faced. The best we can hope for at this point is to limit warming to below the threshold where the carbon-cycle feedbacks kick into overdrive, bringing about catastrophe (80 feet of sea level rise, widespread desertification, >50% species loss).

In all likelihood we need to slow cut emissions deeply and quickly enough that we get to the point this century where we can actually have net negative emissions, by removing carbon dioxide from the atmosphere while emitting almost none.


20 Responses to “Stabilizing climate requires near-zero emissions”

  1. David B. Benson says:

    Yes. Now convince the U.S., Chinese and Indian governments.

  2. Paul K says:

    Stable climate doubtless exists within some definably allowable amount of climate change. The article refers to centennial timescales. We are in the midst of a two and a half century long warming. While the warming in the last 25 years of the 20th century was sharp, the century’s was not. Temperatures in this century demonstrate the warming is not arithmetic or congruent with CO2.

    Zero emissions in 100 years may or not be in the realm of science fiction and will certainly depend on cursed yet undeveloped technologies and never been seen international cooperation. I do think we can expect U.S. energy to be mostly carbon free in as little as 30 – 50 years.

  3. Joe says:

    Sorry, Paul K. Human forcings are the main driver of climate change this century.

  4. Jade A. says:

    Unless there’s some sort of global mandate for massive cuts in GHG emissions, we will get no where near the required cuts that are necessary to stave off a a cataclysmic disaster. According to this post , even if we have a national solar energy plan, close down and forbid any new coal fire power plants worldwide, switch to ethanol, end industrial scale deforestation, we are still going to be in for a rude awakening from mother nature. The only plausible solution is Geo-Engineering. I mean why not? If human induced global warming is causing, and will continue to cause severe weather anomalies, then we should be investing heavily in research for Geo-Engineering.

  5. Joe says:

    Jade — it won’t be easy. Geo-Engineering could possibly be a small part of the solution, but only if we make deep emissions cuts first. Salon will be publishing an article (not by me) on this shortly, and I’ll do a post about it at that time.

  6. Jonas says:

    Hi, there are two technologies that yield negative emissions.

    They are quite new, but are receiving great interest from the scientific community and the engineering world.

    -The first concept is called biochar (recently called “revolutionary” by the UNCCD)

    -The second is coupling biomass power plants to carbon capture and storage (already being trialled in several places).

    Both concepts yield carbon-negative bioenergy – that is: they remove CO2 from the atmosphere.

    Either of these two technologies can reverse climate change by mid century (2060). Combined they can do it in a few decades time (at least according to the Abrupt Climate Change Strategy group, which is a mandate from the G8 studying scenarios that require radical emissions cuts.)

  7. Ronald says:

    Paul K,
    I think the idea of the last few decades increase is significant is because the rate of change is so fast. It’s not unlike the time value of money, If you keep money in the bank long enough, earning compound interest for enough time and wait long enough or never die, you can accumulate a lot of money over time. We keep adding more carbon dioxide to carbon dioxide that is already causing warming and accumulating, it makes for an accumulating rate of warming. It doesn’t matter that the warming rate was low in the early part of the century, it only matters how much more carbon dioxide we keep adding that adds to increased warming.

    To whoever want’s to answer it,
    Are all the scientists in on some colossal joke that some psychologists have dreamed up to see how society handles a problem situation? Like some flight simulator, once the crew in the cockpit has handled an emergency situation, make the problem worse to see if they can handle the pressure. Is that it, huh? The goal line always keeps moving, just when we have some thinking of what the goal should be, they add another unachievable goal.

    I remember in Army Basic training, the drill sergeant would always say on the 18 mile marches, ‘the hike is over when we hike past those trees’ and then, ‘it’s over when we get over that hill’ and does that 50 times just to mess with us. That’s what this is, right?

  8. Paul K says:

    If human forcings are the main driver of climate change this (21st) century, the effect has been negligible so far. If you mean the 20th, it is only the last 25 years as climate science says the 1900 – 1945 warming and the 1945 – 1975 cooling were driven by other factors. Saying temperature is not congruent with CO2 does not dispute a correlation, only that they do not proceed in lockstep.

    I also am 100% behind replacing carbon energy and think it can be done relatively quickly in this country.

  9. David B. Benson says:

    Paul K — Not negligible in the oceans: temperature, acidification and dead zone expansion.

  10. Earl Killian says:

    It hardly seems like you need a journal article to tell us we need to eventually get to zero emissions to stabilize things. The atmospheric concentration is the integral of our emissions, so any positive value eventually causes problems. Isn’t the whole idea of large cuts (60% or 80%) by 2050 simply to slow the problem down to survive the consequences so that post 2050 we can find a way to get to zero? Eventually we may want to add a small negative emissions component for some period of time.

    The Vostok record suggests that sources and sinks were roughly in balance for the last 600,000 years (the 180ppm minima and the 300ppm maxima weren’t changing much from one cycle to the next, the cycles being due to feedbacks amplifying orbital changes). We have conducted a simple experiment in the last few hundred years that essentially proves that as you increase emissions, the sinks cannot keep up (otherwise CO2 concentrations would not exceed the 600,000 maximum by over 30% with no end in sight for the increase. Therefore net positive emissions will accumulate.

  11. David B. Benson says:

    Earl Killian — The anthropomorphic additions to the active carbon cycle are removed (in the medium term) into the deep ocean. In the long term, via calcification. So given long enough, the additions are eliminated.

    But that is much too long for world civilization to wait for.

    I take the point of the paper is the revision from a target of 80% (or whatever) to 100%. The sooner the better.

    And then, IMO, sequester some additional carbon to cool things off, reverse ocean acidification, regrow glacier ice, etc.

  12. Hal says:

    1. Where is there evidence that a global average CO2 concentration above 300 ppm can be climate-forcing neutral (with all other known critical factors held constant)?

    2. Given that some other known critical factors (e.g., sinks) are not constant and will continue to change toward reducing carbon flux into terrestrial and marine carbon sinks for decades (or longer) due to current accumulated atmospheric carbon (383 ppm global average), how can anything but negative net emissions bring the carbon cycle into balance and return Earth toward a stable climate system again?

  13. Jim says:

    Ratio of available solar energy Venus/Earth: 190%

    Venus receives almost twice as much heat as we do.

    Earth, surface pressure: 1000 mbar; temperature: 288K
    Venus, 50km altitude pressure: 1000 mbar; temperature: 330K
    330K/288K = 114%

  14. Jim says:

    Venus, surface pressure: 90,000 mbar; temperature: 735K
    Temperature of terrestrial air compressed from 288K/1,000mbar to 90,000mbar: 887K
    735K/887K = 82.9%

  15. David B. Benson says:

    Hal — There is no recent (last 600,000 years) paleoclimate data that suggests stability above 300 ppm, AFAIK. Climate models predict an eventual quasi-equilibrium for the climate at every potential level of CO2. We won’t like the long-term results at anything above about 315 ppm CO2, in my (only partly knowledgable) opinion.

    The issue is not eventual stability, but rather restoring the climate to the best state for agriculture, i.e., conditions prevailing during the Holocene rather than those in the just-started Anthropocene.

  16. Beefeater says:

    how can anything but negative net emissions bring the carbon cycle into balance and return Earth toward a stable climate system again?
    Again? When exactly has the earths climate ever been “stable”?
    Who is going to define stability, average or normal?

  17. RhapsodyInGlue says:


    “Jade — it won’t be easy. Geo-Engineering could possibly be a small part of the solution, but only if we make deep emissions cuts first.”

    Is this statement based on something scientific or merely your opinion about political expediency. Why must efforts of “geo-engineering”… such as trying to increase ocean CO2 uptake or enhance the albedo of clouds… only start after deep emissions cuts.

    I think the urgency of deep emissions cuts is extremely serious and should be implemented as fast as possible with a goal of complete decarbonization. However, already the earth’s environment and species are suffering from increased heat. Why not employ whatever technologies we have if they can help in the short term by avoiding ecosystem damage and provide a bit extra margin against hitting tipping points such as massive albedo flip or methane release?

  18. Joe says:

    I’ll do a post on geoengineering when the Salon piece comes out.

  19. David B. Benson says:

    Beefeater — The climate has been remarkably stable throughout the Holocene. During the introduction and spread of agriculture. Now that humans have caused the Anthropocene instead, agriduculture (and lots else) is in big trouble.

  20. Paul K says:

    Is Anthropocene a recent coinage?