Study: Global Warming Causes Most Monthly Heat Records Today

by Dana Nuccitelli, via Skeptical Science

A new paper published in Climatic Change by Coumou, Robinson, and Rahmstorf (CRR13) examines the increased frequency of record-breaking monthly temperature records over the past 130 years, finding that these records are now five times more likely to occur due to global warming, with much more to come.

“…worldwide, the number of local record-breaking monthly temperature extremes is now on average five times larger than expected in a climate with no long-term warming. This implies that on average there is an 80% chance that a new monthly heat record is due to climatic change … Under a medium global warming scenario, by the 2040s we predict the number of monthly heat records globally to be more than 12 times as high as in a climate with no long-term warming.”

Fig 5Figure 1: Observed record ratio (the increase in the number of heat records compared to those expected in a world without global warming) for monthly heat records as it changes over time (thin red line is annual data, thick red line smoothed with half-width 5 years). This is compared with predictions from a simple stochastic model based only on the global mean temperature evolution (blue line with uncertainty band directly comparable to the smoothed red curve)

Data and Methods

CRR13 considers the formula for the probability of a record-breaking extreme in a Gaussian (bell curve) time series with a linear long-term warming trend, compared to the much more simplified version of the same formula when there is no warming trend.  The paper then examines the ratio of those two equations – the increased frequency of record-breaking extreme heat events in a warming world.

The study uses global surface temperature data provided by the NASA Goddard Institute for Space Studies (GISS) for 1880–2010, in 2° by 2° grids across the globe, excluding polar regions above 70° latitude due to the sparse temperature station coverage there  They examine the temperature data for each calendar month of the year.


CRR13 finds that the number of observed heat records is much larger than one would have expected in a climate with no long term warming, and many monthly heat records have been broken over the past decade.

In Figure 2 below, the increase in observed monthly heat records in the past decade over the most recent 40-year period of data (left column) are compared to the modeled results (right column) for northern hemisphere summer (top row), winter (middle row), and the whole year (bottom row).

Fig 3Figure 2: Global maps of the observed record ratio (the increase in the number of heat records compared to those expected in a world without global warming) as observed (left panels) and estimated by the model (right panels) using the 1971–2010 dataset. a and b show boreal summer results (June-July-August), c and d austral summer results (December-January-February) and e and f results for all months.

Figure 3 looks at the increase in heat records over the past decade as compared to the full 131-year dataset. The similarity between Figures 2 and 3 shows that over the past decade, the monthly records in the past decade as compared to the past 40 years are usually also records as compared to the past 131 years.

The bottom right panel (d) also shows the probability that a monthly heat record in a given location is due to global warming, with blue indicating 0% probability and red indicating 100%.

Fig 4

Figure 3: Global maps of the observed record ratio over the past decade (the increase in the number of heat records compared to those expected in a world without global warming) over the 1880–2010 dataset, for a boreal summers (June-July-August), b austral summers (December-January-February) and c all months. d Risk map showing the probability that a record-breaking event in the last decade is due to climatic change.

In Figure 1 above, CRR13 extends the model forward assuming global warming based on a moderate emissions scenario, Representative Concentrations Pathway (RCP) 4.5, in which human greenhouse gas emissions peak around the year 2040, ultimately causing a radiative forcing (global energy imbalance) of 4.5 Watts per square meter in 2100 (a doubling of atmospheric CO2 would cause a forcing of about 3.7 Watts per square meter).  This scenario would ultimately lead to about 3.6°C global surface warming above pre-industrial levels, which is a very dangerous and possibly catastrophic amount of global warming, but certainly not a worst case scenario.  It essentially represents a scenario where we take too-slow and gradual action to reduce human greenhouse gas emissions, and at the moment seems fairly realistic.

In this scenario, CRR13 finds that by 2040, monthly heat records will have become approximately 12 times more likely to occur than in a non-warming world,

“…approximately 80% of the recent monthly heat records would not have occurred without human influence on climate. Under a medium future global warming scenario this share will increase to more than 90% by 2040.”

As lead author Coumou noted, this is even worse than it sounds, because breaking a heat record in 2040 will require much higher temperatures than breaking a record today.

“Now this doesn’t mean there will be 12 times more hot summers in Europe than today – it actually is worse.  To count as new records, they actually have to beat heat records set in the 2020s and 2030s, which will already be hotter than anything we have experienced to date.  And this is just the global average – in some continental regions, the increase in new records will be even greater.”

The results of this research are consistent with those of Hansen et al. (2012), which found that global warming is shifting the temperature distribution to make extreme heat waves more likely to occur, similar to the findings of several other studies such as Donat and Alexander (2012) and Meehl et al. (2009).

Source: NASA/Goddard Space Flight Center GISS and Scientific Visualization Studio

Hot Times Ahead

To sum up the results of this study,

  • Record-breaking monthly temperature records are already occurring five times more often than they would in the absence of human-caused global warming.
  • There is an 80% chance that any monthly heat record today is due to human-caused global warming.
  • Unless we take steps to significantly reduce human greenhouse gas emissions and global warming, by 2040 the frequency of monthly heat records will become 12 times the rate in a non-warming world, and we will be able to blame more than 90% of heat records on global warming.

This would of course be bad news.  For example, as shown by Hawkins et al. (2012), crops tend not to respond well to extreme heat, so these findings could pose a significant problem for global food production, as well as increasing heat fatalities, requiring costly adaptive measures to prepare people for more frequent extreme heat waves.  In January of 2013, Australia has been trying to cope with this sort of extreme heat, which has resulted in devastating wildfires and other nasty consequences.

CRR13 presents a reality which we should try very hard to reverse, and a possible future we need to do our best to avoid.

– This piece was originally published at Skeptical Science and was re-printed with permission.

15 Responses to Study: Global Warming Causes Most Monthly Heat Records Today

  1. wili says:

    Hot times ahead, indeed. On top of all the ghgs we are dumping into the air at ever-greater rates, feedbacks seem to be kicking in with gusto:

    Even more reason to cut back on burning death fuels to try to slow the Climate Blitzkrieg that is now upon us.

    I see from other sites that what looked like a developing El Nino is now morphing back into La Nina, spelling another dry year in the American West and Midwest. So…more mega-fires and more dried-out farms and ranches. Already, some farmers are finding nothing but dry dust as deep as 6 feet.

  2. Jeremy says:

    Dr. James Hansen and his team also produced a similar paper on weather extremes that came to the same finding.
    For those that need visual proof that global warming is here unpon us now go see “Chasing Ice” and film featuring Extreme Ice Survey of James Balog.
    With time lapse photos over a short time one can literally witness the glacier melting away…very quickly… this is way beyond natural cycles.
    How come folks don’t get it? there is a campaign out there so they don’t.
    I feel it is too late now. If we were to address it now, I doubt people would accept the measures.
    Well, maybe I’m wrong.

  3. Superman1 says:

    I’m starting to believe we have been victims of a numbers shell game. Decades ago, 3 C was presumed to be livable. Then two decades ago, international agreements were signed that agreed upon 2 C as a livable target. Now, people like Anderson say 1 C is closer to a livable target. Yet, at 0.8 C, we see the Arctic ice cap about to go in Summer, warm Atlantic waters rushing in to the Arctic to finish the job (reminds me of the HMS Dorsetshire torpedoing the Bismarck after the Swordfish from the Ark Royal disabled the rudder), and the shallow clathrates placed at risk. Was there any basis to these numbers? Looking back, maybe 0.4-0.5 C were the real livable numbers. If this is really the case, and it’s not too late, do we really have any choice other than a full court press on eliminating fossil fuels ASAP, rapid carbon recovery ASAP, low-risk geo-engineering ASAP to ‘quench’ the positive feedback mechanisms we already see in action?

  4. Superman1 says:

    “How come folks don’t get it? there is a campaign out there so they don’t.

    I feel it is too late now. If we were to address it now, I doubt people would accept the measures.

    Well, maybe I’m wrong.”

    Look, I’ve known two people who had lung cancer, had a lobe resected, and then resumed smoking a few weeks after discharge from the hospital. One was in her early forties, the other in his late forties. Both were gone within three years. Some people would rather live the ‘good life’ at the expense of a long life. Unfortunately, that’s where we seem to be as a society, and not only limited to smoking and fossil energy use.

  5. wili says:

    “I’ve known two people who had lung cancer, had a lobe resected, and then resumed smoking a few weeks after discharge from the hospital. One was in her early forties, the other in his late forties. Both were gone within three years.”

    Good point. We probably all know such stories.

    This is why I don’t think one needs to resort to any conspiracy to explain the inaction and silence from our esteemed leaders. They are doing what nearly all the rest of us are doing:

    We know that fairly extreme levels of power-down and conversion to alternative sources of energy are required (or were required twenty or thirty years ago), but we can’t quite face the enormity of the challenge.

    Most airplane trips are not necessary, but do you know of anyone who has foregone a planned trip to avoid contributing to the problem?

    Most urban car trips are less than five miles, well withing the ability of most to walk or bike–but how many people do you know who have given up on their car to avoid contributing to the problem?

    Eating industrial meat is a major contributor to the problem, but how many are going veggie or vegan (or local/organic/grassfed) to reduce their contribution to the problem?

    There are many avenues to activism that could affect policies in the right direction at many levels, but how many are engaging actively in these?

    And I’m talking about even the people who are best informed about the issue.

    Essentially none of us are responding in our personal usage or in our activism at levels anywhere near appropriate to the enormity of the predicament.

    Why should we expect ‘leaders’ to be so far ahead of the rest of us? They have rarely been so in the past.

  6. John McCormick says:

    Almost 60 inches of recorded rain in 4 days. La Nina and El Nino appear to be off track. We are witnessing the unraveling of our natural system into chaos. More rain in Australia. Less rain in the North American grain belt. Can food riots be on the horizon? Yes!

  7. Jim Baird says:

    Wili and Superman, I believe you are both right. Greed and fear are the only motivators and fear isn’t working. As Kerry points out however energy is a $6 trillion market. With OTEC you can service and capitalize on this market while mitigating the damage that has been built in.

  8. Rabid Doomsayer says:

    How about a realistic worst case scenario with a time frame. Then compare with where we are now.

    Do you think we can give the end Permian a run for nastiness? The PETM was a little nasty, but the end Permian; that was nasty.

  9. Jeremy says:

    Even if we were able to cut our emissions by 75% in the year 2060 in the USA, that still would not be enough. That savings of 75% would only be transfered to the have nots.
    Remember, the world stands to have 9 billion people! When i was born in circa 1960, there were only 3 billion of us.
    Also, the worls will be at “peak everything”, putting more pressure to use all available resources.
    Not a pretty picture.
    Something got to give!

  10. Jan says:

    I foresee an inevitable acceptance of our plight by virtually everyone, even the politicians, because Pearl Harbor moments will expand to be the norm, as ave. temps grow inevitably to 3-6 deg F, barring a Superman moment..

    By, then, it will pretty much be political pandemonium. A good percentage of our population will simply be unable to live and to consume energy, so the emissions will peak. But, at what tragic cost, one can not imagine – any more than one cold “imagine” nuclear war.

  11. Mulga Mumblebrain says:

    Indeed! The days of El Nino and La Nina are over, and I like to call the new weather pattern, La Bruja, after my primary school head-mistress.

  12. Mulga Mumblebrain says:

    It already has, old chap! We’ve fallen off the cliff, and the rocks beckon. Levitation from here will be, to put it mildly, challenging.

  13. Hops says:

    Regarding the assertion that we could walk or bike a few miles rather than drive, I’ve heard that it takes so much fossil fuel to produce a calorie of food that driving is actually more efficient. It actually seems credible.

    Of course, if you had to walk to the store, you’d probably make fewer trips…

  14. Mulga Mumblebrain says:

    That may be true of industrialised Agribusiness, but grow your own or buy local, organic stuff, and eat within season, and the effect will disappear.