By Stephen Smith, Scientist at the CCC
“Action to reduce emissions only makes sense if the world really is warming, and if our emissions are the main cause. How confident in these two propositions can we be? Many newspaper articles and internet blogs would suggest the jury is still out. But two new scientific studies provide fresh and independent evidence of the reality of human-induced warming.
“Warming of the climate system is unequivocal” and “most of the increase… is very likely due to the observed increase in anthropogenic greenhouse gas concentrations”. These are conclusions agreed by leading climate scientists in the latest assessment from the Intergovernmental Panel on Climate Change.
Outside of the expert community, public debate is as active as ever. Controversy tends to focus on the global average surface temperature record – the iconic indicator of climate change, but not the only one – estimated from thermometer readings around the world. Critics claim that the warming shown in estimates from three different research centres is simply an artefact of biased sampling, poor quality records, and growing Urban Heat Islands (UHIs).
Scientists counter that they have already accounted for these potential errors. Even so, the claims troubled Richard Muller, a physics professor from Berkeley, California, enough to assemble a team of eminent scientists to investigate for themselves. First results were released in October and cover the land surface only (not the ocean, where UHI bias is minimal). While only preliminary, they show remarkable agreement with the other existing estimates (Fig 1).
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Figure 1: Results from BEST compared to pre-existing datasets.
Source: http://berkeleyearth.org/analysis.php
The evidence for surface warming since the mid-20th Century would seem robust. But what about the second proposition: are we to blame?
This is where the second new study comes in. Previous studies have attempted to answer the question by comparing observations against the warming patterns produced by climate models run with and without human factors. Instead of taking the same approach, this study employs the powerful principle of conservation of energy. Simply put, the heating power of all the different drivers of climate change (natural and human) must be balanced by energy going into the oceans and out of the atmosphere into space. Applying this constraint to a climate model, scientists have been able to show how much of the warming since the 1950s is due to each known factor (Fig 2).
Figure 2: Contributions of factors to surface warming since the 1950s; whiskers denote 5-95% uncertainty range. Bars on right show the sum of all anthropogenic and natural (solar and volcanic) drivers. Dashed lines show observed warming from the three datasets in Fig 1 (colour matched). Grey shaded region shows the 5-95% range of natural variability in climate models (i.e. undriven by human, solar or volcanic factors).
Source: modified from Huber & Knutti (2011) Anthropogenic and natural warming inferred from changes in Earth’s energy balance, Nature Geoscience
Their conclusions closely match those of previous attribution studies and add further detail: it is 95% likely that at least three quarters of the observed warming is due to human activity.
In science, conclusions become stronger when they are independently verified by other scientists, and when multiple different lines of evidence point to the same result. It seems unlikely this new evidence will change the minds of diehard critics. But proper scepticism is providing a clearer picture of human-induced warming.