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Climate Variability and Climate Change: The New Climate Dice
10 November 2011
J. Hansen, M. Sato, R. Ruedy
Abstract.
The "climate dice" describing the chance of an unusually warm or cool season, relative to the climatology of 1951-1980, have progressively become more "loaded" during the past 30 years, coincident with increased global warming. The most dramatic and important change of the climate dice is the appearance of a new category of extreme climate outliers. These extremes were practically absent in the period of climatology, covering much less than 1% of Earth's surface. Now summertime extremely hot outliers, more than three standard deviations (σ) warmer than climatology, typically cover about 10% of the land area. Thus there is no need to equivocate about the summer heat waves in Texas in 2011 and Moscow in 2010, which exceeded 3σ ? it is nearly certain that they would not have occurred in the absence of global warming. If global warming is not slowed from its current pace, by mid-century 3σ events will be the new norm and 5σ events will be common.
The greatest barrier to public recognition of human-made climate change is the natural variability of 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?
This question assumes great practical importance, because of the need for the public to appreciate the significance of human-made global warming. Actions to stem emissions of the gases that cause global warming, mainly CO2, are unlikely to approach what is needed until the public perceives that human-made climate change is underway and will have disastrous consequences if effective actions are not taken to short-circuit the climate change.
Early recognition of climate change is critical. Stabilizing climate with conditions resembling those of the Holocene, the world in which civilization developed, requires that rapid reduction of fossil fuel emissions begin soon (Hansen et al., 2011).
The Goddard Institute for Space Studies (GISS) surface air temperature analysis (Hansen et al., 2010) is carried out at two spatial resolutions: 1200 km and 250 km. In this paper we use the 250 km analysis, which is well-suited for our objective to illustrate seasonal-mean climate variability on regional spatial scales and investigate the significance of recent extreme events.
Summer temperature anomalies. 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 colder 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.
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