Climate change certainty

So how certain is climate change, really?

Turns out the global scientific community is a lot more certain than you might expect, given how much climate change gets denied in politics.

Here’s the basic run-down of what we know:

• Global warming has been happening — and the evidence is unequivocal.
• Human activities have definitely contributed to global warming.
• It’s 95% certain that human activities have caused most of the global warming since 1950.
• The best estimate is that human activities have caused all or nearly all of the global warming since 1950.

Or to sum up in a single sentence: Global warming is unequivocally happening, and humans have caused most — maybe all — of it.

Continue reading for details and citations.

The Intergovernmental Panel on Climate Change (IPCC) has published consensus documents called Assessment Reports, most recently in 2014. These Assessment Reports are the work of hundreds of scientists, carefully reviewing and synthesizing the available literature. In total, these reports distill the work of thousands of scientists from countries around the globe. The IPCC reports deliberately err on the side of caution, highlighting the uncertainties in the current scientific knowledge.

The IPCC is extremely clear on their main point that global warming is happening:

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased. [^1]

That’s right folks, global warming is unequivocal. There is no uncertainty or wiggle room in that statement.

Individual aspects of climate change have different degrees of evidence and support for them, ranging from likely to virtually certain. In aggregate, however, they add up to the simple, unequivocal fact: global warming is happening.

And people are causing it:

Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes.

It is a simple fact — again, with no uncertainty or wiggle room — that human activities have had an influence on climate change. We have contributed to atmospheric and oceanic warming, sea level rise, and the other aspects listed here.

That still leaves open the question of how big our contribution has been. Here, the scientific consensus puts in a few qualifiers:

It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

The term “extremely likely” is defined by the IPCC to mean 95-100% certainty; the only higher term is “virtually certain” (99-100%). The term “dominant cause” gets explained more in the subsequent text:

It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period.

These two sentences are essentially an estimate with a confidence interval. The best estimate is that human activities have caused all or nearly all of the post-1950 global warming. But a quantitative estimate almost always comes with some uncertainty. In this case, the scientific consensus is 95% certain that human activities have caused at least half of the global warming.

This statement focuses on the period since 1950, when anthropogenic greenhouse-gas emissions and global warming both rose sharply, compared to the more gradual increases in both since 1750. We also have much better climate records from this more recent, well-instrumented, period. For both of these reasons, there’s much more signal-to-noise, so it’s easier to be certain about post-1950 global warming.

I’d like to point out that the question of what caused a past event is a very tricky one. There is always going to be some uncertainty in the answer, because we don’t get to replicate history, nor can we run controlled experiments on it. Further studies will narrow the range of reasonable possibilities, but it’s inherently unlikely that we’ll ever have unequivocal evidence about the cause of historical events.

Global warming is unequivocally happening, and humans have caused most — maybe all — of it. Anyone who argues otherwise is denying the facts.

For further reading beyond the IPCC, NASA has compiled a list of climate-change position statements from various scientific societies.

 

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