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More Carbon Emissions = Less Global Warming?

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400 PPM: What’s Next for a Warming Planet
Concentrations of greenhouse gases in the atmosphere have reached this level for the first time in millions of years. What does this portend? »

It’s the debate du jour in the climate blogosphere: How much hotter will the planet get as carbon dioxide levels in the atmosphere rise? The contention began July 20, when the Economist reported that a yet-to-be-released table of data from the Intergovernmental Panel on Climate Change (IPCC), showed that global temperatures would not rise as high this century as scientists had previously predicted.

For years scientists have said that if atmospheric levels of carbon dioxide reach 450 parts per million (ppm) the planet would heat up by an average of 2 degrees Celsius above pre-industrial levels. They have also said that if the world crosses that threshold, ecosystems worldwide would suffer serious damage. The IPCC data, however, indicated that levels of 425 to 485 ppm would correspond only to a temperature increase of 1.3 to 1.7 degrees C by 2100.

That’s a big difference. The Earth has warmed by about 0.8 degree C since the start of the Industrial Revolution, when the CO2 count was 280 ppm. This past May the Earth hit the 400-ppm mark for the first time in 800,000 years, and the average level is now climbing at about 2 ppm annually. Falling short of 2 degrees C by 0.3 degree or more could delay damaging effects. However, the notion of diminished warming is dubious at best. If anything, the world could hit the 2-degree threshold sooner than it thinks.

After the Economist published the table, which it had somehow obtained, IPCC spokespeople and independent experts quickly trashed the data—and the magazine itself. They said the table was a preliminary draft, was not reviewed, was not even being used by the relevant IPCC experts, and should never have been quoted.

The IPCC stands by its equation of 450 ppm = 2 degrees C, based on “climate sensitivity” data—how much temperatures will rise given an ongoing increase in atmospheric carbon dioxide. And indeed, prior IPCC temperature projections have held up. Those projections matter because the desire to avoid warming of more than 2 degrees C has driven policy decisions in many countries and underlies international climate agreements.

Those agreements notwithstanding, global greenhouse gas levels continue to rise. Industrialized countries actually cut their CO2 emissions by 7 percent in the past five years, according to the Worldwatch Institute, but emissions from developing countries have soared. The total global output keeps increasing, hitting a new record high of 31.6 million metric tons in 2012.

Furthermore, recent science shows that two major mechanisms that tend to slow the rise of atmospheric CO2 are diminishing. First, over the past decade the oceans have absorbed about 26 percent of all CO2 emissions, according to the Scripps Institution of Oceanography. About 28 percent is taken up by plants, leaving 46 percent wafting into the atmosphere. But scientists from various institutions have noted that the rate of ocean absorption is slowing. There are several reasons why, but part of the effect is simply that the more the water absorbs, the harder it becomes to store even more. The atmosphere will likely take up a larger share of emissions—unless we plant lots of trees.

Second, as the world shifts away from burning coal and reduces tailpipe emissions, those steps lessen the amount of aerosols—tiny particles—released into the atmosphere. We benefit with less pollution and fewer health problems, but aerosols also block incoming sunlight; fewer particles will allow more solar heating.

Some experts point out that we’ve already hit the 450-ppm mark. Technically speaking, the level is called “450-ppm equivalent”—the warming from all greenhouse gases, such as methane, expressed as if all the heating were due to CO2. Estimates indicate that when CO2 levels alone reach about 400 ppm, the total effect of all gases would be about 450-ppm CO2 equivalent. Having hit 400 ppm of CO2 for a brief time in May (the high point each year), we’re essentially at that 450-ppm equivalent now.

It is still difficult to say how much temperatures will rise by 2050 or 2100 due to the carbon dioxide that is already in the atmosphere—known as “the warming in the pipeline.” There is a lag between any rise in CO2 levels and the heating that results, so the planet is “locked in” to further warming—and to the chief repercussions such as further sea level rise. But the IPCC has released good estimates of the pipeline: the best case is that the average global temperature at the Earth’s surface will rise 1.5 degrees C by 2100, compared with 1990 levels. The worst case is 4.5 degrees C, and the most likely case is 3 degrees C.

In his own assessment of the numbers, Dana Nuccitelli, a physicist who writes at the Skeptical Science blog—known for deep analysis of these matters—notes that the 1.5 degrees C case would only be possible if the world stopped increasing emissions by 2020 and then began reducing them by 3.5 percent a year. As he notes, that scenario “involves extremely aggressive greenhouse gas emissions reductions.”

Aggressive is not impossible, although it may be unlikely. Still, anything that the world does to reduce emissions slows the eventual temperature rise. Scientific American has written about many possible steps, including a succinct short-list by my colleague David Biello.

We’ll have an updated—and fully peer-reviewed—analysis of warming soon; the IPCC will release its next big assessment of CO2 data and all things climate on September 23.

Mark Fischetti About the Author: Mark Fischetti is a senior editor at Scientific American who covers energy, environment and sustainability issues. Follow on Twitter @markfischetti.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. sethdiyal 2:30 pm 07/30/2013

    Note that the antinuke Germany is on track to increase its deadly air pollution and GHG emission two years in a row. Always ready to sacrifice millions of lives in pursuit of lofty goals – this time a silly dream of a world powered by cool breezes and warm sunbeams.

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  2. 2. M Tucker 2:37 pm 07/30/2013

    Mark, you did see the latest EIA (Energy Information Administration) report for International Energy Outlook through 2040 didn’t you? Well the world in not in any position to “reduce emissions.” The world will continue to depend on and increase emissions from coal, natural gas, and ‘liquids’ well past 2040 as the report illustrates. So I would go with the worst case scenario of the FINAL IPCC report.

    As for the 2 degree Celsius threshold…let’s make a list of the scientists who think that 2 degrees is some sort of safe line in the sand. Please post the list. Are they saying that 1.9 is OK but 2 is disastrous? OK, how about 1.7 is that remarkably different from 2? How about 1.5? Where is the ‘safe’ range if 2 represents “suffering?”

    Have you heard of Dr Chris Field? He is the Director of the Carnegie Institution’s Department of Global Ecology at Stanford. Actually he is the founding director. He recently won the Max Planck Research Prize for “significantly [increasing] our knowledge of how life on Earth responds to climate change.” The Planck Prize is kind of a big deal with a larger cash prize than the MacArthur Genius Grants. Hers is what Dr Field said about the 2 degree threshold:

    “Politically, 2C might be a useful target to rally the global community around,” said Chris Field, the director of the Carnegie Institution’s Department of Global Ecology at Stanford. “But the concept of a safe threshold is a myth and tends to distract attention from evidence that we are already seeing widespread and consequential impacts of climate change.”

    “We’re already experiencing real impacts from climate change that we need to cope with today,” he said. “But a frustratingly large amount of the dialogue on climate change risks pointing people away from smart solutions by making the problem seem either simpler or more complicated than it really is.”

    “We’re already seeing evidence of climate-change impacts in the increased frequency of extreme events. We’ve seen record-setting temperatures almost every year, including a phenomenal number in the United States in 2012. Globally, nine of the 10 hottest years on record have occurred since 2000. In the future, high-temperature episodes are very likely to become more frequent and more severe. For most areas on land, we are already seeing that, and expect a future with more of the rainfall coming in the heaviest events, the kind of events that can lead to floods.

    “In the last century, sea level has risen an average of about 6 inches. Over the rest of the century, it could rise another 12 to 30 inches. If you’re trying to manage risk and prevent disasters, it’s important to recognize that the damages will continue to occur in the extremes. Not acting magnifies risk in the same way as not wearing a seat belt or not having insurance.”

    “The idea that we won’t see meaningful impacts of climate change until we reach warming of 2C is demonstrably wrong. We’ve already experienced widespread and consequential impacts of climate change.

    “The 2C threshold may be a useful policy target, but that’s different from recognizing it as some kind of guardrail where we know we’re safe if we don’t pass it. There are almost certainly tipping points in the climate system, levels of warming beyond which Earth experiences really major impacts. One of these would be the threshold for eventual melting of a major continental-scale ice sheet like Greenland, which has an ice volume representing more than 20 feet of sea-level rise. It is possible that the threshold for commitment to melting of the Greenland ice sheet is close to 2C, but it might be substantially lower or higher. We really don’t know what that temperature threshold is.”

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  3. 3. emily127 2:50 pm 07/30/2013

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  4. 4. Postman1 2:53 pm 07/30/2013

    “Second, as the world shifts away from burning coal…”
    What ‘world’ is that?
    China, India, Brazil, Germany, and others are increasing their use of coal according to various articles on this very site.

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  5. 5. dana1981 5:22 pm 07/30/2013

    Minor correction – though I have a masters degree in physics, I’m an environmental scientist and climate blogger, not a physicist.

    -Dana Nuccitelli

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  6. 6. rshoff 8:11 pm 07/30/2013

    There is no tenable solution to sustain our current lifestyle. More nuclear waste is not a solution, it’s just more waste.

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  7. 7. Gizmo 9:37 pm 07/30/2013

    More nuclear waste is not just a solution, it’s free fuel for traveling wave reactors.


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  8. 8. Dr. Strangelove 2:33 am 08/5/2013

    “The IPCC stands by its equation of 450 ppm = 2 degrees C, based on “climate sensitivity” data — how much temperatures will rise given an ongoing increase in atmospheric carbon dioxide.”

    It must be a political equation plus a lot of hand-waving. What does the latest science say? For doubling of CO2: 560 ppm = 2.0 degrees C

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