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The case for global warming is stronger than ever

Despite what many deniers, aided by an irresponsible media who have no ability to distinguish a legitimate expert from a crank, want you to believe, the case for global warming has never been stronger.

A recent study by the Met Office makes it clear that the evidence supporting global warming is getting better all the time:

By looking at a wide range of observations from all over the world, the Met Office study concludes that the fingerprint of human influence on climate is stronger than ever. “We can say with a very high significance level that the effects we see in the climate cannot be attributed to any other forcings [factors that push the climate in one direction or another],” says study co-author Gabriele Hegerl of the University of Edinburgh…

The fact that climate change evidence that was “very likely” a few years ago [the the IPCC AR4]  has now been declared likelier still by the comprehensive Met Office report suggests that the evidence for human-caused climate change is getting better all the time.

Or in other words, the small uncertainty that exists regarding climate change is getting smaller all the time, despite non-scientific claims to the contrary.

Here is the abstract:

Detection and attribution of climate change: a regional perspective

Peter A. Stott, Nathan P. Gillett, Gabriele C. Hegerl, David J. Karoly, Daithi A. Stone, Xuebin Zhang and Francis Zwiers

The Intergovernmental Panel on Climate Change fourth assessment report, published in 2007 came to a more confident assessment of the causes of global temperature change than previous reports and concluded that ‘it is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent except Antarctica’. Since then, warming over Antarctica has also been attributed to human influence, and further evidence has accumulated attributing a much wider range of climate changes to human activities. Such changes are broadly consistent with theoretical understanding, and climate model simulations, of how the planet is expected to respond. This paper reviews this evidence from a regional perspective to reflect a growing interest in understanding the regional effects of climate change, which can differ markedly across the globe. We set out the methodological basis for detection and attribution and discuss the spatial scales on which it is possible to make robust attribution statements. We review the evidence showing significant human-induced changes in regional temperatures, and for the effects of external forcings on changes in the hydrological cycle, the cryosphere, circulation changes, oceanic changes, and changes in extremes. We then discuss future challenges for the science of attribution. To better assess the pace of change, and to understand more about the regional changes to which societies need to adapt, we will need to refine our understanding of the effects of external forcing and internal variability.

One of the more interesting figures of the report (bellow) shows the expected surface temperatures trends for natural facings alone (green), and both natural and anthropogenic forgings (red). Consistently the observed temperature trends fall within the expected trends caused by anthropogenic and natural forcing, with a few regions having trends that fall outside the bell curve or probable trend values obtained with just natural forcing:

Click for to embiggen

Looking at the above figure one sees that Western North America, the Pacific, the Atlantic, Northern Europe, the Mediterranean Basin, South Asia, North Australia, and South Australia, all display temperature trends that are either completely outside the bell curve of temperatures expected for natural forcings alone, or right on the fringe. In fact only Central and Eastern North America, have trends that might be expected to occur with only natural forcings. But in these regions the differences between natural and anthropogenic trends are quite small, and the observed temperatures are still well within the anthropogenic bell curve.

And when looking at the globe as a whole one gets temperatures that fall just outside of the natural bell curve, and well within the anthropogenic curve:

And it is worth noting that the top of the anthropogenic curve (representing the mean median and mode) is less than the observed trend, though not by much.

The study concludes:

The wealth of attribution studies reviewed in this article shows that there is an increasingly remote possibility that climate change is dominated by natural rather than anthropogenic factors. Progress since the AR4 has shown that discernible human influence extends to reductions in Arctic sea ice and changes in the hydrological cycle associated with increasing atmospheric moisture content, global and regional patterns of precipitation changes, and increases in ocean salinity in Atlantic low latitudes. In addition, changes in Antarctic temperatures (the one continent on which an attribution study was not available at the time of AR4) have been attributed to human influence and there is increasing evidence that human influence on temperature is becoming significant below continental scales, as would be expected from the large-scale coherence of surface temperature. We have discussed in this review how attributed changes in atmospheric moisture content and precipitation patterns are consistent with theoretical expectations.

While deniers are busy inventing one scandal after another, the science keeps chipping away at the uncertainty that remains. The picture of the climate system has not changed, it has only become more clear.

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