James Hansen On The New Climate Dice And Public Perception Of Climate Change

Earth’s Northern Hemisphere over the past 30 years has seen more “hot” (orange), “very hot” (red) and “extremely hot” (brown) summers, compared to a base period defined in this study from 1951 to 1980. This visualization shows how the area experiencing “extremely hot” summers grows from nearly nonexistent during the base period to cover 12 percent of land in the Northern Hemisphere by 2011. Watch for the 2011 heat waves in Texas, Oklahoma and Mexico, or the 2010 heat waves the Middle East, Western Asia and Eastern Europe. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

By James Hansen, Makiko Sato, Reto Ruedy, via NASA’s Goddard Institute for Space Studies

The greatest barrier to public recognition of human-made climate change is probably the natural variability of local climate. How can a person discern long-term climate change, given the notorious variability of local weather and climate from day to day and year to year?

The question is important because actions to stem emissions of gases that cause global warming are unlikely until the public appreciates the significance of global warming and perceives that it will have unacceptable consequences. Thus when nature seemingly provides evidence of climate change it needs to be examined objectively by the public, as well as by scientists.

Therefore it was disappointing that most early media reports on the heat wave, widespread drought, and intense forest fires in the United States in 2012 did not mention or examine the potential connection between these climate events and global warming. Is this reticence justified?

In a new paper (Hansen et al., 2012a), we conclude that such reticence is not justified. The paper attempts to illustrate the data in ways that properly account for climate variability yet are understandable to the public.

We show how the probability of unusually warm seasons is changing, emphasizing summer when the changes have large practical effects. We calculate seasonal-mean temperature anomalies relative to average temperature in the base period 1951-1980. This is an appropriate base period because global temperature was relatively stable and still within the Holocene range to which humanity and other planetary life are adapted (note 1).

We illustrate variability of seasonal temperature in units of standard deviation (σ), including comparison with the normal distribution (“bell curve”) that the lay public may appreciate. The probability distribution (frequency of occurrence) of local summer-mean temperature anomalies was close to the normal distribution in the 1950s, 1960s and 1970s in both hemispheres (Fig. 2). However, in each subsequent decade the distribution shifted toward more positive anomalies, with the positive tail (hot outliers) of the distribution shifting the most.

Figure 2. Temperature anomaly distribution: The frequency of occurrence (vertical axis) of local temperature anomalies (relative to 1951-1980 mean) in units of local standard deviation (horizontal axis). Area under each curve is unity. Image credit: NASA/GISS.

An important change is the emergence of a subset of the hot category, extremely hot outliers, defined as anomalies exceeding +3σ. The frequency of these extreme anomalies is about 0.13% in the normal distribution, and thus in a typical summer in the base period only 0.1-0.2% of the globe is covered by such hot extremes. However, we show that during the past several years the global land area covered by summer temperature anomalies exceeding +3σ has averaged about 10%, an increase by more than an order of magnitude compared to the base period. Recent examples of summer temperature anomalies exceeding +3σ include the heat wave and drought in Oklahoma, Texas and Mexico in 2011 and a larger region encompassing much of the Middle East, Western Asia and Eastern Europe, including Moscow, in 2010.

The question of whether these extreme hot anomalies are a result of global warming is often answered in the negative, with an alternative interpretation based on meteorological patterns. For example, an unusual atmospheric “blocking” situation resulted in a long-lived high pressure anomaly in the Moscow region in 2010, and a strong La Niña in 2011 may have contributed to the heat and drought situation in the southern United States and Mexico. However, such meteorological patterns are not new and thus as an “explanation” fail to account for the huge increase in the area covered by extreme positive temperature anomalies. Specific meteorological patterns help explain where the high pressure regions that favor high temperature and drought conditions occur in a given summer, but the unusually great temperature extremities and the large area covered by these hot anomalies is a consequence of global warming, which is causing the bell curve to shift to the right (Fig. 2).

Yet the distribution of seasonal temperature anomalies (Fig. 2) also reveals that a significant portion (about 15 percent) of the anomalies are still negative, corresponding to summer-mean temperatures cooler than the average 1951-1980 climate. Thus people should not be surprised by the occasional season that is unusually cool. Cool anomalies as extreme as -2σ still occur, because the anomaly distribution has broadened as well as moved to the right. In other words, our climate now encompasses greater extremes.

Our analysis is an empirical approach that avoids use of global climate models, instead using only real world data. Theories for the cause of observed global temperature change are thus separated as an independent matter. However, it is of interest to compare the data with results from climate models that are used to simulate expected global warming due to increasing human-made greenhouse gases.

Indeed, the “climate dice” concept was suggested in conjunction with climate simulations made in the 1980s (Hansen et al., 1988) as a way to describe the stochastic variability of local temperatures, with the implication that the public should recognize the existence of global warming once the dice become sufficiently “loaded” (biased). Specifically, the 10 warmest summers (Jun-Jul-Aug in the Northern Hemisphere) in the 30-year period (1951-1980) were used to define the “hot” summer category, the 10 coolest the “cold” category, and the middle 10 the “average” summer. Thus it was imagined that two sides of a six-sided die were colored red, blue and white for these respective categories. The divisions between “hot” and “average” and between “average” and “cold” occur at +0.43σ and -0.43σ for a normal distribution.

Temperatures simulated in a global climate model (Hansen et al., 1988) reached a level such that four of the six sides of the climate dice were red in the first decade of the 21st century for greenhouse gas scenario B, which is an accurate approximation of actual greenhouse gas growth (Hansen and Sato 2004; updates are provided by a Columbia Univ. webpage). Observed summer temperature anomalies over global land during the past decade averaged about 75% in the “hot category”, thus midway between four and five sides of the die were red, which is reasonably consistent with expectations.

Figure 3. Frequency of occurrence (vertical axis) of local June-July-August temperature anomalies (relative to 1951-1980 mean) for Northern Hemisphere land in units of local standard deviation (horizontal axis). Temperature anomalies in the period 1951-1980 match closely the normal distribution ("bell curve", shown in green), which is used to define cold (blue), typical (white) and hot (red) seasons, each with probability 33.3%. The distribution of anomalies has shifted to the right as a consequence of the global warming of the past three decades such that cool summers now cover only half of one side of a six-sided die, white covers one side, red covers four sides, and an extremely hot (red-brown) anomaly covers half of one side. Image credit: NASA/GISS.

The relation between the bell curve and climate dice is illustrated in Fig. 3. Extremely hot outliers already occur more frequently than unusually cold seasons. If the march of the bell curve to the right continues unabated, within a few decades even the seasons that were once considered average will cease to occur.

We have shown that the increased frequency of “hot” seasons is a result of global warming. The cause of global warming is a separate matter, but observed global warming is now attributed with high confidence to increasing greenhouse gases (IPCC 2007a).

Both attributions are important. Together they allow us to infer that the area covered by extreme hot anomalies will continue to increase in coming decades and that even more extreme outliers will occur. Indeed, we conclude that the decade-by-decade shift to the right of the temperature anomaly distribution (Fig. 2) will continue, because Earth is now out of energy balance, with more solar energy absorbed than heat radiation emitted to space (Hansen et al., 2011); it is this imbalance that drives the planet to higher temperatures. Even an exceedingly optimistic scenario for fossil fuel emissions reduction, 6%/year beginning in 2013, results in global temperature rising to almost 1.2°C relative to 1880-1920, which compares to a current level ~0.8°C (Hansen et al., 2012b).

Figure 4. Wildfire frequency and spring-summer temperature in the western United States. Image credit: Westerling et al. (2006).

Practical effects of increasingly loaded climate dice occur mainly via amplified extremes of Earth’s water cycle. The broadening of the “bell curve” of temperature anomalies is related to interactions of warming with the water cycle. Hot summer anomalies occur when and where weather patterns yield an extended period of high atmospheric pressure. This condition is amplified by global warming and the ubiquitous surface heating due to elevated greenhouse gas levels, thus increasing the chances of an extreme anomaly. Yet global warming also increases atmospheric water vapor overall, causing, at other times or places, more extreme rainfall and floods, consistent with documented changes over Northern Hemisphere land and the tropics (IPCC 2007b).

The (Northern Hemisphere) summer of 2012 is still unfolding. A global map of the anomaly distribution will be provided on aColumbia Univ. webpage) once the data are complete; the data so far suggest that parts of the United States and Asia likely will be in the extreme (+3σ) category. One of the consequences of extreme summer heat anomalies is increased area and intensity of wildfires, as shown in Fig. 4. Updates of these data and other climate impacts after the 2012 data are complete will be useful for assessing impacts of continued global warming.

James Hansen, Makiko Sato, and Reto Ruedy, via NASA’s Goddard Institute for Space Studies

20 Responses to James Hansen On The New Climate Dice And Public Perception Of Climate Change

  1. fj says:

    The absolute reality and calls for immediate action are completely rational and relentless. Resistance is futile; and much worse.

  2. Figure 2 is remarkable! I plan to use it in my next talk.

  3. Dan Miller says:

    To put it in even simpler terms, Extremely Hot Summers are now occurring 50 to 100 times (5000% to 10,000%) more often than just 50 years ago. That means when a new 3-sigma event happens (like this summer’s Midwest heat wave), the probability that it was caused by natural variation is 1/100 to 1/50 (1 to 2 percent) and the chance it was caused by global warming is 49/50 to 99/100 (98 to 99 percent). The probability that a string of these events occurring (like we have recently seen) is due to natural variation is vanishingly small.

    P.S. The probabilities come from the fact that 50 years ago, 0.1% to 0.2% of the globe experienced 3-sigma events on average. Last decade (2001-2011), about 10% of the globe experienced these events.

    P.P.S. Dr. Hansen said in his press conference that they excluded urban temperature stations from the study in order to avoid the urban heat island issue.

  4. Leif says:

    Indeed, resistance is fatal!

  5. Dan Miller says:

    A few more points on this amazing study:

    (1) It is not based on climate models. It is just a simple statistical analysis of actual, measured temperature readings from the past 60 years (1951-2011).

    (2) It is not a prediction! The 5000% to 10,000% increase in Extremely Hot Summers already happened!

    (3) It is the 3-sigma events that cause death and destruction. The 2003 European heat wave killed over 50,000 people and the 2011 Texas drought caused over $7 billion in damage.

  6. Doug Grandt says: is required reading for red Congress-foot-draggers who say “we don’t have the luxury to address environmental issues during this sour economy because it will hurt small business and poor constituents.” Luxury? We are running out of time. If Congress is impotent in stalemate, then let’s write more and compel Rex Tillerson to change course. WRITE REX, WRITE AWAY! If Rex leads on a carbon fee (which he claims to support), Congress may follow out of shame, especially if Rex leads in making Tyrannosaurus Rex-finery extinct, beginning with one T Rex in 2013, the new beginning.

  7. prokaryotes says:

    Isn’t it possible to just model the uptake of weather extremes when applying the moderators of climate forcings to the baseline of an equilibrium climate state?

    Although to some degree single events can be pinpointed to certain anomalies, like polar air intrusion, ocean current alteration or magnitudes. But basically everything is interconnected and since there is more energy around, all events are biased/favored to the extremes.

    I wonder if the timing of changes – the frequency of a period when the weather swings back to the other extreme is also changing, like amplifying. Ofc events seem to be more persistent too but changes during temperature swings appear to be more abrupt and severe. A bit like an patients heart beat which runs faster…

  8. Brian R Smith says:

    Putting the research in simple terms, at every opportunity, as you have, is darned important, and clarifying variability is an important target.

    I’ve read the paper and while I ken what Hanson et. al. mean by, for example, “A normal distribution of variability has 68 percent of the anomalies falling within one standard deviation of the mean value.”..the general public will mostly glaze over and head for the summary. Which matters not because in the end most people will either take the climate scientists word for it or not based on giving credibility to the science. And Hanson simplifies beautifully when he speaks, steadily gaining credibility for the science.

    At the top of the post he emphasizes that

    “actions to stem emissions of gases that cause global warming are unlikely until the public appreciates the significance of global warming and perceives that it will have unacceptable consequences.”

    This is the nut of the problem. The quickest path to mass public recognition of the unacceptable consequences, I think, is a presentation to the nation by a cohort of climate scientists and climate leaders in a national town hall meeting on the net and every MSM channel arrangeable. If the credibility of the science, and the absurdity and harm of the disinformation campaign, can both be established in one stroke, it would put us miles ahead in bolstering all current campaigns and handing Obama loud, visible support for showing leadership.

  9. Merrelyn Emery says:

    With no disrespect to N. Hemispherians, there is life below the equator, W Antarctica is one of the fastest warming points on the globe and the Great Southern has a huge impact on global weather systems. An occasional mention of the planetary system and global situation would help redress the current overemphasis on the North, ME

  10. prokaryotes says:

    Yesterday in Italy 47C also heat is spreading now to Spain..

    Heat wave, torrential rain make for dangerous mix in Italy–torrential-rain-make-for-dangerous-mix-in-Italy.html

    And a bit farther north in Germany 19C today…

  11. Leif says:

    Thanks for the T-link Doug, I will get one off.

    Bumper sticker of the day:

    Go Solar, Resistance is Fatal.

  12. prokaryotes says:

    High temperatures scorch Balkans economies
    posted 9 hours ago.

    The heatwave is far from just a Romanian phenomenon. Even as the US Midwest is ravaged by drought, unusually hot and dry weather has affected much of Europe’s prime agricultural land – from eastern Italy across the Balkans and Black Sea, and into Russia as far as western Siberia and in Kazakhstan.
    It has caused power shortages, led to more than 300 wildfires in Bulgaria, and produced an algae epidemic in the Sea of Azov – an offshoot of the Black Sea in south-east Ukraine – which witnesses said turned blood-red.
    Cities have seen temperatures close to records since June or early July.
    Temperatures in Budapest, the Hungarian capital, this week hit 37.7C, topping the previous record of 35.8C set in 1939. Kiev, Ukraine’s capital, reached 37.4C, its second-highest temperature at any time for 112 years, a fraction below the 2010 record.
    Also this week, Podgorica, capital of Montenegro, hit 43.5C – the highest since records began 50 years ago. The government advised citizens to stay indoors between 10am and 6pm.
    In the Balkans and Ukraine, the heatwave follows one of the harshest winters in memory, with temperatures dropping below minus 30C, heavy snowfalls and parts of the Black Sea freezing over.

  13. Aleph Null says:

    Hansen’s full paper links to a website with more charts on southern hemisphere data.

  14. Merrelyn Emery says:

    Thank you Aleph, ME

  15. John McCormick says:

    Brian, no argument with your comment.

    Though I found Dr. Hansen’s “unacceptable consequences” too weak to stir average public into action.

    The consequences are playing out right now, every day and every way. That many Americans right now are finding the heat and drought unacceptable is certain. For the rest of us not trapped parched, burning lands, we are not living in “unacceptable” times. Sure, it is hot and straight line winds are blowing down tress and power lines but that is a temporary inconvenience.

    What Dr. Hansen and the scientists fail to spell out is the raw truth that the consequences will sweep past the “unacceptable” stage and eventually become fatal to our way of life.

    Perpetual droughts in the NA midwest, Colorado River becoming a trickle in late summer; Phoenix a ghost town with its 125 degree temperatures the norm, NYC swamped by the Hudson and East Rivers.

    Chaos, that is the future and “unacceptable” is a non-sequitur when it think of that consequence of a 2 degree temperature increase. Does anyone doubt we are going to reach that level?

  16. Mark E says:

    “Changing baselines”, in my opinion, ties with the disinformation campaign as the second-biggest obstacle to public understanding.

  17. Spike says:

    Yes it shows up the “global warming stopped in 1998” crowd as dolts.

  18. Brian R Smith says:

    I don’t doubt we will reach that level and you rightly characterize what’s coming as Chaos. Deep social chaos that will be impossible to manage (away from violence and suffering) without strong moral and practical leadership from government, exactly what we do not have.

    So who is in a position to crack this thing open, to tell the truth to the public, expose the seriousness of harm from dis-informers, take the lead on policy for a job-creating renewables transition, and make climate reality a litmus test for
    candidates? It is the whole group of climate leaders, from science to community, acting together, who have that power. To me this implies a responsibility to develop a collaborative strategy. Particularly a media strategy. Climate strategy summit. Sooner the better.

  19. Tom Lent says:

    Chaos is an important term to use in describing this. Climate change does not sound anywhere near as dramatic as what we are seeing. Global warming doesn’t match people’s reality when they experience a record cold winter. Hansen’s plots point to the increase in chaos in the climate with 3-sigma events that bring the long term devastation of sea level rise into more short term focus through extreme heat, wildfires and hurricanes. So my suggestion is to start referring to “climate chaos” and figure out a layperson friendly way of describing what a 3-sigma event means.