In a report released just before Christmas, the UK Met Office lowered its decadal forecast for the expected average global temperature. The press release noted that:
Global average temperature is expected to remain between 0.28 °C and 0.59 °C (90% confidence range) above the long-term (1971-2000) average during the period 2013-2017, with values most likely to be about 0.43 °C higher than average. The warmest year in the 160-year Met Office Hadley Centre global temperature record in 1998, with a temperature of 0.40°C above long-term average. The forecast of continued global warming is largely driven by increasing levels of greenhouse gases.
The revision was initially ignored in the Christmas rush, but with the festive season now over, the story has reappeared. Some media outlets interpreted this as evidence that "global warming had stopped", given that the medium term forecast was no different to the temperature peak seen in the late 1990s. One particular columnist caused the Met Office to release a point-by-point rebuttal of his claim that the Office was "useless".
Despite the acrimony, the revision does raise the question as to what is happening. On the one hand we are seeing an increase in the number of severe heat events globally, yet on the other there has been seemingly little change in global average temperature for much of the last decade.
The starting point must always be the fact that the increase in CO2 in the atmosphere will create a global heat imbalance, at least until a new steady state is reached (e.g. through changes in cloud cover, surface albedo etc. ). That steady state will also take many centuries to reach, given the huge inertia in the climate system. Current estimates put the size of the imbalance at about 3 W/m2 (Hansen et. al., 2009), which although small compared to the total heat arriving from the sun is significant compared to swings over the past million years that have resulted in large shifts in planetary ice cover.
The imbalance is offset to a degree by the effect of aerosols, which scatter incoming solar radiation and therefore act as coolants. There remains considerable uncertainty in the science community regarding the extent of the aerosol impact and how it might be changing over time. For example, the recent (10 years) sharp increase in coal use in China, much of which does not have sulphur emission handling, may well be adding enough sulphur (an efficient coolant) in the atmosphere to dampen the warming trend that would otherwise be seen. The charts below show the various forcings and the net effect. The large error bar illustrates the uncertainty linked with aerosols, to the extent that the red line (GHGs) and blue line (Aerosols) could cancel if at the extremes of their respective ranges.
The proxy we use to "measure global warming" is the surface temperature record, because both a recorded history and derived history of this measurement exists and because it's relatively easy to take the necessary measurements. In the case of the recorded history, it is typically 100-150 years, but in the UK it starts in 1659 (1772 for the daily series). But real "global warming" is far broader than this and includes ocean heating (surface and deep ocean) and land ice melting.
As the additional heat building up in the atmosphere distributes through the ocean/ice/atmosphere system it is unlikely that a uniform and unchanging temperature rise in one particular part of this system would be the result. The interaction between them and the impact of short term aerosols will likely result in volatility in the surface temperature record. This has been seen before, most recently in the post war period when temperature remained flat for about 20 years. Some have attributed this to the aerosol loading from the rapid increase in coal burning in the USA and Europe over that period, none of which had sulphur scrubbing. As sulphur emissions fell sharply with the arrival of scrubbers, so the masking effect was removed and temperatures began rising.
To simply argue that "global warming has stopped" is short sighted. The evidence to support such a claim is not there.
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