Perceptions of climate change: the new climate dice

“Climate dice,” describing the chance of unusually warm or cool seasons relative to climatology, have become progressively “loaded” in the past 30 years, coincident with rapid global warming. The distribution of seasonal mean temperature anomalies has shifted toward higher temperatures and the range of anomalies has increased. An important change is the emergence of a category of summertime extremely hot outliers, more than three standard deviations (σ) warmer than climatology.

This hot extreme, which covered much less than 1% of Earth’s surface in the period of climatology [1951-1980], now typically covers about 10% of the land area. We conclude that extreme heat waves, such as that in Texas and Oklahoma in 2011 and Moscow in 2010, were “caused” by global warming, because their likelihood was negligible prior to the recent rapid global warming. We discuss practical implications of this substantial, growing climate change.

That’s the finding of a detailed climatological analysis by NASA’s James Hansen along with Makiko Sato and Reto Ruedy in which they attribute some of the uber-extreme heat waves to global warming.

The entire Hansen et al paper is a must-read. The authors explain why they focus on summer:

Summer, when most biological productivity occurs, is the most important season for humanity and thus the season when climate change may have its biggest impact. Global warming causes spring warmth to come earlier and it causes cooler conditions that initiate fall to be delayed. Thus global warming not only increases summer warmth, it also protracts summer-like conditions, stealing from both spring and fall. Our study therefore places emphasis on study of how summer temperature anomalies have been changing.

The paper also explains the ‘dice’ metaphor and why they are not fans of using a new climatological period, such as 1981-2010 in place of 1951-1980. I will excerpt some key parts and post some key figures.

First, you may be wondering why the top chart of summer hot area percentage doesn’t have as clear a trend for the United States as it does for North America or the globe. As the authors explain:

The small area of the contiguous 48 states (less than 1.6% of the globe) causes temperature anomalies for the United States to be very “noisy”. Nevertheless, it is apparent that the long-term trend toward hot summers is not as pronounced in the United States as it is in hemispheric land as a whole. Also note that the extreme summer heat of the 1930s, especially 1934 and 1936, is comparable to the most extreme recent years.

Year-to-year variability, which is mainly unforced weather variability, is so large for an area the size of the United States that it is perhaps unessential to find an “explanation” for either the large 1930s anomalies or the relatively slow upturn in hot anomalies during the past few decades. However, this matter warrants discussion, because, if the absence of a stronger warming in recent years is a statistical fluke, the United States may have in store a relatively rapid trend toward more extreme anomalies.

Some researchers have suggested that the high summer temperatures and drought in the United States in the 1930s can be accounted for by sea surface temperature patterns plus natural variability (10, 11). Other researchers (12-14), have presented evidence that agricultural changes and crop failure in the 1930s contributed to changed surface albedo, aerosol (dust) production, high temperatures, and drying conditions. Furthermore, both empirical evidence and climate simulations (14, 15) indicate that agricultural irrigation has a significant regional cooling effect. Thus increasing amounts of irrigation over the second half of the 20th century may have contributed a summer cooling tendency in the United States that partially offset greenhouse warming. Such regionally-varying effects may be partly responsible for differences between observed regional temperature trends and the global trend.

They explain the “loaded climate dice” metaphor:

“Loading” of the “climate dice” describes the systematic shift of the frequency distribution of temperature anomalies. Hansen et al. (2) represented the climate of 1951-1980 by colored dice with two sides colored red for “hot”, two sides blue for “cold”, and two sides white for near average temperatures. With a normal distribution of temperatures the dividing point would be at 0.43σ to achieve equal (one third) chances of being in each of these three categories in the period of climatology (1951-1980).

A climate model was used (2) to project how the odds would change due to global warming for alternative greenhouse gas scenarios. Scenario B, which had climate forcing that turned out to be very close to reality, led to four of the six dice sides being red early in the 21st century based on global climate model simulations.

Fig. 5 confirms that the global occurrence of “hot” anomalies (seasonal mean temperature anomaly exceeding +0.43σ) has approximately reached the level of 67% required to make four sides of the dice red, with the odds of either an unusually “cool” season or an “average” season now each approximately corresponding to one side of the six-sided dice. However, the loading of the dice over land area in summer is even stronger (Fig. 5, lower row).

Probably the most important change is the emergence of a new category of “extremely hot” summers, more than 3σ warmer than climatology. For practical purposes it is important to look at the changes over land areas, where most people live, rather than the global mean for which anomalies are more constrained by the ocean’s thermal inertia. Fig. 6 illustrates that +3σ anomalies practically did not exist in the period of climatology (1951-1980), but in the past several years these extreme anomalies have covered of the order of 10% of the land area.

… Warming is larger in winter than in summer, but this tends to be more than offset by the much larger natural variability in winter (Fig. 2), which makes it harder for the public to notice climate change in winter. Another factor affecting the public’s perception of winter warming is the fact that snowfall amounts increase with global warming (in regions remaining cold enough for snow), and there is a tendency of the public to equate heavy snowfall and harsh winter conditions, even if temperatures are not extremely low.

The increase, by more than a factor 10, of area covered by extreme hot anomalies (> +3σ ) in summer reflects the shift of the anomaly distribution in the past 30 years of global warming, as shown succinctly in Fig. 4. One implication of this shift is that the extreme summer climate anomalies in Texas in 2011, in Moscow in 2010, and in France in 2003 almost certainly would not have occurred in the absence of global warming with its resulting shift of the anomaly distribution. In other words, we can say with a high degree of confidence that these extreme anomalies were a consequence of global warming….

More at the link