På NOAAs webb Climate.gov ställer man sig frågan (November 8, 2013) Why did Earth’s surface temperature stop rising in the past decade?
The most likely explanation for the lack of significant warming at the Earth’s surface in the past decade or so is that natural climate cycles—a series of La Niña events and a negative phase of the lesser-known Pacific Decadal Oscillation—caused shifts in ocean circulation patterns that moved some excess heat into the deep ocean.
The “pause” in global warming observed since 2000 followed a period of rapid acceleration in the late 20th century. Starting in the mid-1970s, global temperatures rose 0.5 °C over a period of 25 years. Since the turn of the century, however, the change in Earth’s global mean surface temperature has been close to zero. Yet despite the halt in acceleration, each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.
Frågorna blir givetvis om det inte var ”natural climate cycles” som
- orsakade temperaturhöjningen 1980-2000
- gjorde att temperaturen inte steg mellan 1940-1980
- gjorde att den steg 1910-1940
During the last decade, a longer than usual solar minimum cycle, several volcanic eruptions, and relatively low amounts of water vapor in the stratosphere may have helped cool the atmosphere temporarily. But recent research suggests that the Earth’s natural climate variability—natural, short-term fluctuations in the climate system that occur on a year-to-year basis or longer—may have played the most pivotal role of all by transferring excess heat from the Earth’s surface into the deep ocean.
One of the most well-known natural climate oscillations—the El Niño-Southern Oscillation (ENSO) cycle—causes swings in sea surface temperatures in the tropical Pacific. Although ENSO originates in the equatorial Pacific Ocean, a strong El Niño or La Niña event is capable of bumping global temperatures upward or downward for a year or two. Since the last major El Niño event in 1997-1998, a series of La Niña events have dominated the tropical Pacific, resulting in a prolonged cooling of sea surface temperatures that has also likely stalled the rise in global temperatures.
Det är ju synd att NOAA inte visar El Niño-perioder före 1960. Än så länge är det oklart vilken inverkan El Niño har på klimatets långsiktiga utveckling.