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It’s “Extremely Likely That at Least 74% of Observed Warming Since 1950″ Was Manmade; It’s Highly Likely All of It Was

By Joe Romm on December 5, 2011 at 7:45 pm

"It’s “Extremely Likely That at Least 74% of Observed Warming Since 1950″ Was Manmade; It’s Highly Likely All of It Was"


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Back in 2009, NASA’s Gavin Schmidt was asked, “what percentage of global warming is due to human causes vs. natural causes?”  His answer:

Over the last 40 or so years, natural drivers would have caused cooling, and so the warming there has been … caused by a combination of human drivers and some degree of internal variability. I would judge the maximum amplitude of the internal variability to be roughly 0.1 deg C over that time period, and so given the warming of ~0.5 deg C, I’d say somewhere between 80% to 120% of the warming. Slightly larger range if you want a large range for the internal stuff.

Turns out he was spot on.

A new study in Nature Geoscience, Anthropogenic and natural warming inferred from changes in Earth’s energy balance” (subs. req’d) finds:

Our results show that it is extremely likely that at least 74% of the observed warming since 1950 was caused by radiative forcings, and less than 26% by unforced internal variability. Of the forced signal during that particular period, 102% (90–116%) is due to anthropogenic and 1% (−10 to 13%) due to natural forcing….  The combination of those results with attribution studies based on optimal fingerprinting, with independent constraints on the magnitude of climate feedbacks, with process understanding, as well as palaeoclimate evidence leads to an even higher confidence about human influence dominating the observed temperature increase since pre-industrial times.

Here’s a figure from the study comparing the magnitude of different “forcing agents” or contributors to warming since the 1950s:

Contributions of different forcing agents to the total observed temperature change. Error bars denote the 5–95% uncertainty range. The grey shading shows the estimated 5–95% range for internal variability. Observations are shown as dashed lines.

The Nature News and Scientific American stories have had misleading headlines:

Three-Quarters of Climate Change Is Man-Made

That’s not a good headline.

The 74% or “three quarters” probability is where the 95% confidence level is for this one study.  As climatologist Kevin Trenberth put it in an email, it is “highly likely” that all of the warming since 1950 is due to human activity:

The study shows that it is highly likely that all of the observed warming over the past 50 years was caused by human activities and, while natural variability could have contributed up to about a quarter of the amount over any comparable period, the recent slowing in global temperature rise suggests that natural variability has contributed to offset the human-induced component if anything.

Climatologist Michael Mann emails me:

Natural forcing over this period was negative. So the most likely conclusion is that anthropogenic forcing explains more than 100% of the observed warming. There is an internal variability component too, but no reason to believe that it has been positive.

Absent the increasing GHGs, we probably would have cooled, since

  1. We’ve had a couple of big volcanoes.
  2. We’re just coming off “the deepest solar minimum in nearly a century.”
  3. The underlying long-term trend had been cooling (see Human-caused Arctic warming overtakes 2,000 years of natural cooling, “seminal” study finds).

Here’s more background on the study:

Knutti and Huber found that greenhouse gases contributed 0.6–1.1 °C to the warming observed since the mid-twentieth century, with the most statistically likely value being a contribution of about 0.85 °C. Around half of that contribution from greenhouse gases — 0.45 °C — was offset by the cooling effects of aerosols….

The authors calculated a net warming value of around 0.5 °C since the 1950s, which is very close to the actual temperature rise of 0.55 °C observed over that period. Changes in solar radiation — a hypothesis for global warming proffered by many climate sceptics — contributed no more than around 0.07 °C to the recent warming, the study finds.

To test whether recent warming might just be down to a random swing in Earth’s unstable climate — another theory favoured by sceptics — Knutti and Huber conducted a series of control runs of different climate models without including the effects of the energy-budget parameters. But even if climate variability were three times greater than that estimated by state-of-the-art models, it is extremely unlikely to have produced a warming trend as pronounced as that observed in the real world, they found.

“This tightens estimates of past responses,” says Gabriele Hegerl, a climate scientist at the University of Edinburgh, UK, “And it should also lead to predictions of future climate change that are grounded in the kind of changes already being observed.”

A key point of the study is that our actual confidence that humans are warming the planet is stronger than the confidence just from this one study.

Last year, Time magazine reported on a comprehensive new review paper of “100 peer-reviewed post-IPCC studies” in an article titled, “Report: The Case for Global Warming Stronger Than Ever” noting:

By looking at a wide range of observations from all over the world,  the Met Office study concludes that the fingerprint of human influence on climate is stronger than ever. “We can say with a very high significance level that the effects we see in the climate cannot be attributed to any other forcings [factors that push the climate in one direction or another],” says study co-author Gabriele Hegerl of the University of Edinburgh.

It is a “settled fact” that the climate system is warming, as the National Academy concluded in 2010.  It is now beyond a reasonable doubt that humans are responsible for most of that warming, and  highly likely we are responsible for all of it.

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24 Responses to It’s “Extremely Likely That at Least 74% of Observed Warming Since 1950″ Was Manmade; It’s Highly Likely All of It Was

  1. oggy bleacher says:

    I’ve got a question: If between 10,000 B.C and pre-industrial (1850) 100% of global warming was naturally occurring then how can 100% of warming be caused by man from 1950-today? Seems speculative to erase any natural warming effects, not that the cause for alarm should be ignored.

    • Mark Shapiro says:

      Man had some effect even before 1850, mostly through agriculture and deforestation. The amount of man’s forcing has been growing since then.

      The study above explicitly does account for natural effects, both positive and negative. End result: man’s effect is very large and growing.

    • Note that it is “warming” we are talking about. ALL of the warming is man made, or even more, because without man earth would be cooling! If the term was “change”, then human contribution would be less in percentage – but how to you measure that?

      Simply said:
      We were “lucky” to have natural factors and our own pollution work to cool the planet. So overall, things should cool. But things didn’t cool, things got hotter. ALL of that, plus some, was likely caused by our green house gas emissions. Thus, >100% is possibly our fault.

    • Joe Romm says:

      Not clear that 100% of it was. Some research suggests otherwise.

    • bones288 says:

      You’ve answered your own question, really.
      The what we’ve been seeing since then, especially in the last several decades, is beyond the range of the previous 10,000 years before that.

      The climate does get naturally ‘forced’ into either a thermally positive or negative directions. And ff we’ve been in a negative slope for the past several decades (or longer), it’s really going to hit the fan if/when it turns around.

    • will says:

      I think the answer is in the question. It’s not that 100% of all warming was since 1950, it’s that 100% of all warming since 1950 was manmade. It doesn’t mean there was no warming before 1950….

  2. fj says:

    Hurricane Irene was NYC’s ghost of climate change future when it nearly flooded the entire subway system.

    Columbia University Earth Institute professor Klaus Jacob: “Had it not been 3.6 feet but 4.6 feet we would have been in deep trouble.”

    Jacob calculated that it would take 29 days to get the subway working almost halting the city’s economy which produces $4 billion a day in economic activity.

    It’s estimated that $15 billion is required to prepare the subway for the inevitable.


  3. SG says:

    @oggy – Go all the way back to the top of the article: “Over the last 40 or so years, natural drivers would have caused cooling…”

    That’s how all of the warming COULD be man made — because without us, the planet would probably have been going through a cooling cycle.

  4. Rabid Doomsayer says:

    It is probable that 120% of the warming is man made. Without us it would have cooled.

    The confidence levels expressed by scientists is not the same as Uncle Fred’s hot tips. Ask a scientist if the sun will come up tomorrow, they will only say “almost certainly”.

    Rabid Doomsayer might be a little careless about how he uses language, but he is in no way a scientist.

  5. james corbett says:

    Are there still deniers out there? I mean beyond the seven dwarfs running for the GOP nomination.

    • Merrelyn Emery says:

      Unfortunately yes James. Here on the table lands in SE Australia we are just about to record the lowest temp records for the beginning to summer ever. If we don’t make a max of 26C in the next few days we’ll break the record and as I sit here doing a perish, (i.e. it is freezing cold by our standards) and given the forecasts, it seems likely we will break this record.

      So what do I hear on the radio while I work? Mainly comments like “Well those climate scientists have a lot to answer for – I am going out to buy a CO2 generator!”

      And I only listen to the ABC – commercial radio is bad for my blood pressure, ME

  6. Dr.A.Jagadeesh says:

    I entirely agree with the post.

    Dr.A.Jagadeesh Nellore(AP)India
    E-mail: anumakonda.jagadeesh@gmail.com

    • Merrelyn Emery says:

      Dr Jagadeesh, I notice your comments.

      I have never felt more like being at home than when in India and so did my kids. Is it possible for you to say more about what is happening in India re the up take of renewables, nuclear and fossil fuels? And how you see those options changing India?

      Thank you for consideration of these questions, ME

  7. Tomas Solfaro says:

    We don’t need to argue about whether climate change is man-made we agreed that it is in a legal binding treaty the UNFCCC. Can’t people read the clear words of article 2? This article establishes a legal fact!

    Follow http://climatechange.blogs.fr

  8. Anna Haynes says:

    In the paper do they estimage the *range* of unforced internal variability’s likely (in the generic sense of the term) contribution to global temp change? (i.e., from a high of 26% of the warming, to a low of…a negative %?)

  9. Anna Haynes says:

    Analogy/communication help needed – what’s the simplest intuitive way to explain why the statement “we’re almost certain that x% is human-caused” does NOT translate to “we’re (equally) almost certain that “(100-x)% is natural”?

    • Anna Haynes says:

      I mean, speaking statistically, irrespective of what the %s are about. (i.e. to explain that just because you’re sure that x% of something is Z, doesn’t mean (100-x)% of that thing is not-Z)