In a world of complete knowledge, we could measure annual temperatures around the globe and then factor out all of the short-term changes not driven by human emissions. What’s left would be the anthropogenic or human-caused global warming trend.
In the real world, however, we don’t measure temperatures everywhere — we interpolate between temperature stations. And we don’t know the exact magnitude of all the short-term natural variability. That leaves plenty of room for global warming deniers and delayers to intermittently push their siren song of “global cooling” (see “Media enable denier spin: A (sort of) cold January doesn’t mean climate stopped warming“). And siren song it is (see “Hadley Center to delayers: We’re warming, not cooling” and “Hansen throws cold water on cooling climate claim” and “Breaking News: The Great Ice Age of 2008 is finally over — next stop Venus!“).
In this post, I will examine some of the factors that have affected recent temperature records (including changes in solar irradiation) to understand what is really going on. First, RealClimate’s Gavin Schmidt has extracted the the El Ni±o – Southern Oscillation (ENSO) signal from the global temperature data (both NASA’s and Hadley’s):
Gosh, it’s warming. Who would have thunk it?
[For more on the ENSO factor, time series smoothing, and global warming, read "Has the world cooled since 1998?" (pp. 141-148).]
You may wonder why NASA shows more warming in recent years than the Hadley Center. The answer is quite important to this discussion. As Schmidt explains, the Arctic Ocean has no temperature station coverage:
[Hadley] does not extrapolate past the coast, while [NASA] extrapolates from the circum-Arctic stations — the former implies that the Arctic is warming at the same rate as the rest of the globe, while the latter assumes that the Arctic is warming as fast as the highest measured latitudes.
Since the Arctic appears to be warming up at an alarming rate — far faster than the rest of the globe, as the literature had predicted — I find the NASA data more compelling. Indeed, if anything, even the NASA dataset probably underestimates the recent warming, since the temperature above an ice-covered Arctic Ocean can get very, very low (many tens of degrees below 0ºC), whereas the temperature above open ocean (or thin ice) is close to 0ºC).
Significantly, The ENSO is not the only ocean-based source of temperature fluctuation:
THE PACIFIC DECADAL OSCILLATION
The PDO is not as well understood as ENSO, but it may be as important. Here is the the PDO’s monthly values since 1900:
Note: William Patzert of NASA’s Jet Propulsion Lab commented on Andy Revkin’s recent post, “Climate Trends With Some ‘Noise’ Removed,” that “the PDO index above is the result of analysis in which any long-term climate trend is removed.”
JPL’s Patzert and Willis (here) explain that
We still don’t really know what causes the PDO. We can see it in the data, but we don’t have a physical explanation for what is driving it. Until we do, we can’t really say what the interactions between global warming and the PDO are.
That said, during long periods of negative PDO, the rate of global warming is slower, and during positive periods of the PDO, the rate of warming is faster.
If we are entering a negative PDO, that could slow the rate of global warming. As Willis and Patzert note, notwithstanding the natural short-term oscillations in the temperature record, the overall temperature increase is pretty obviously caused by human activities. Here is the full NASA record:
HERE COMES THE SUN?
The favorite denier notion that the Sun is responsible for the global warming of the past century. That myth has been long debunked — see see “Fred Thompson, Global Warming Denier and Sun Worshiper” and Skeptical Science and AMS Seminar and RealClimate. Indeed, a major 2007 study in the Proceedings of the Royal Society concluded:
Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth’s climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.
That said, the Sun does have a well-known short-term irradiance cycle. As NASA explains:
The sun is another source of natural global temperature variability. Figure 3, based on an analysis of satellite measurements by Richard Willson, shows that 2007 is at the minimum of the current 10-11 year solar cycle. Another analysis of the satellite data (not illustrated here), by Judith Lean, has the 2007 solar irradiance minimum slightly lower than the two prior minima in the satellite era.
Figure 3. Solar irradiance from analysis of satellite measurements by Willson and Mordvinov (Geophys. Res. Lett. 30, no. 5, 1199, 2003) and update (private communication). Click to enlarge.
This cyclic solar variability yields a climate forcing change of about 0.3 W/m2 between solar maxima and solar minima…. Several analyses have extracted empirical global temperature variations of amplitude about 0.1°C associated with the 10-11 year solar cycle, a magnitude consistent with climate model simulations….
The solar minimum forcing is thus about 0.15 W/m2 relative to the mean solar forcing. For comparison, the human-made greenhouse gas climate forcing is now increasingly at a rate of about 0.3 W/m2 per decade. If the sun should remain ‘stuck’ in its present minimum for several decades, as has been suggested in analogy to the solar Maunder Minimum of the seventeenth century, that negative forcing would be balanced by a 5-year increase of greenhouse gases. Thus such solar variations cannot have a substantial impact on long-term global warming trends.
The solar cycle has dropped temperatures about 0.1°C in the past decade. ENSO added about 0.15°C to the 1998 peak. The PDO may be having an effect now but its magnitude is uncertain. If we subtracted out all of that “noise,” human-caused global warming appears to be about 0.1°C to 0.2°C a decade as predicted.
The greenhouse gas forcing will increasingly dwarf the natural forcings in the coming years and decades. Absent a major volcano, we will probably see record temperature growth in the 2010s, as two recent major studies concluded, see “Nature article on ‘cooling’ confuses media, deniers: Next decade may see rapid warming” and “Climate Forecast: Hot — and then Very Hot.” Then the “global cooling” meme will disappear forever, to be replaced by the “global desperation” meme, which I call Planetary Purgatory.
This is particularly true if the Arctic goes ice-free in the next decade, which seems increasingly likely. And if that causes accelerated methane emissions from the tundra, then all model forecasts of future temperature rise go up in flames (see “Tundra 4: Permafrost loss linked to Arctic sea ice loss“).
If the short-term temperature fluctuations of the last few years lead us to another decade of inaction, then we are headed to 1000 ppm this century — and that means warming for the rest of this century will average more than 0.5°C a decade (see “Nature publishes my climate analysis and solution” and “Is 450 ppm politically possible? Part 0: The alternative is humanity’s self-destruction“). The implications are staggering: By mid-century, the planet would be warming each decade as much as it warmed in the past half century.
The time to act is yesterday.