Let me start by giving you a little quiz. Some area of the world has a record multi-day heat wave. The media and some scientists suggest that anthropogenic (human-caused) climate change contributed. What percent of the extreme temperature is caused by (1) human-enhanced greenhouse gases (GG) or (2) natural variability (NV)?
a. 100% GG and 0% NV
b. 90% GG and 10% NV
c. 0% GG and 100% NV
d. 10% GG and 90% NV
e. 50% of each
The answer in most cases would probably be (d). The largest contribution to extreme temperatures is from natural variability, which is not the impression one usually gets from the media. Rather, the implication of most stories is that human-caused increases in greenhouse gases, such as CO2, are the overwhelming contributors to extreme weather events. Natural variability causes temperature changes that would have occurred without any human impacts.
The Golden Rule
Considering the substantial confusion in the media about this critical issue, let me provide the GOLDEN RULE OF CLIMATE EXTREMES. Here it is:
The more extreme a climate or weather record is, the greater the contribution of natural variability.
Or to put it a different way, the larger or more unusual an extreme, the higher proportion of the extreme is due to natural variability.
The Golden Rule of Climate Extremes is very different from the implications of many media stories, which suggest that a highly unusual event is mainly the result of anthropogenic global warming.
It also has a very important implication. Since the major contributor to extremes is natural, most large climate-related impacts (e.g., heat wave and drought over California last year) would have been occurred with or without human enhancement of greenhouse gases.
Now before I get some folks concerned or upset, let me be clear that I am not some kind of global warming denier. Global warming due to greenhouse gas increases IS occurring now. But in virtually all situations its amplitude today is much smaller than natural variability. THAT'S the point.
It is easy to demonstrate the GOLDEN RULE OF CLIMATE EXTREMES.
Here is the IPCC global temperature future projections assuming a large increase of greenhouse gases (A2 scenario). About a 1C warm up so far globally.
Of course, the warming is not uniform, with the Arctic warming up the most and continents warming more than oceanic regions. This is illustrated in the projected change between 1980-1999 and 2020-2029 from the IPCC report. The Arctic is forecast to warm by 2-3C.
Now consider the heat wave of winter 2015. Here is the difference from normal (1990-1995) for January-May (the temperature anomaly). Huge warmth in the western U.S., with anomalies reaching 4-5C. The global warming signal is much less (perhaps 1-1.5C). So most of the warmth must be from natural variability. And we even know what natural variability that did the deed: unusual ridging (high pressure) over the western U.S. and the eastern Pacific.
The record breaking precipitation this last winter over the Pacific Northwest? Global warming suggests only a very minor increase in winter precipitation, completely dwarfed by the huge wet anomaly this winter. Natural variability rules, at least for now.
Another way to understand the Golden Rule of Climate Extremes is to look at probability plots of temperatures. Here is an example from the IPCC Science report. Temperature probability plots tend to be Gaussian (bell-shaped), with the highest probability near the mean. The gray line shows the situation before any anthropogenic warming. The probabilities decrease towards warm and cold extremes, with this distribution caused by natural variability.
If the mean warms, the whole distribution tends to shift towards warmer temperature (dashed line). The probability of extremes increases, but the reason they are much warmer than the new mean is STILL because of natural variability. Even if there was no global warming, the extreme temperatures far to the right of the mean would STILL be extreme.
Let me put it a different way. If there was no natural variability, NO ONE would be talking about heat waves or precipitation extremes. If temperatures, were always the same and warmed up by a 1-2C, few would notice. Our temperatures typically reach around 44F in midwinter. If they rose to a steady 47F, would you even notice?
A frequent analogy for anthropogenic climate change is that it is like putting the climate system on steroids. But think about this comparison for a second. Steroids incrementally improve the performance of world-class athletes. They are already 90% of the way there and they are looking for a small additional edge (which is huge when you are playing at their level). You don't give steroids to the average person and expect they will be breaking world records. Similarly, without natural variability doing most of the work, you don't get extreme weather.
Now some folks might ask: couldn't global warming cause some kind of climate discontinuities, whereby the modest radiative effect from CO2 causes a jump in temperatures or a radical reorganization of the atmosphere. Such a hypothesis was the basis for the movie, A Day After Tomorrow.
For most of the planet, this does not seem to be the case. Our best models do NOT suggest it. In fact, there is substantial research that suggests that variability in the atmosphere could deamplify as the planet warms.
As the Earth warms, the global warming signal will increase progressively and eventually will produce temperature anomalies in some location as large as those produced by natural variability today. But these is not the case now and won't be for a long time (end of century). So remember the GOLDEN RULE OF CLIMATE EXTREMES and hopefully some of the media will keep it in mind.
Several folks have noted that global warming will increase the frequency of extremes. That is absolutely true. But nevertheless most of the origin of the extremes will still be from natural variability. Consider a population of people in which the heights range natural from 60 to 79 inches. So there is a natural variation of heights from 5 feet to 6 ft 5 inches. Now a new vitamin/protein supplement is discovered that increases the heights of people by 1 inches. There will now be new record heights (6 ft 6 inches). A big increase in frequency for such heights. But most of the variability (now from 61 to 80 inches) will still be natural. Climate is the same way.