Is the world’s climate changing and is it our fault?

Let’s check the science: Is it true that human activity is driving serious global climate change?

I’ve been aware of the potential dangers of global warming since the late 1970’s. Since that time, I’ve seen the scientific evidence for the consequences of burning fossil fuels accumulate and I’ve become increasingly concerned. But in the media, I encounter people (admittedly none of them with any expertise in the field), who vehemently deny that the situation exists. They do not convince me, but they make me wonder how anyone can hold such positions in the face of overwhelming evidence. In an attempt to appreciate the magnitude of their foolishness, I think it’s time to remind myself how convincing the evidence for human-induced climate change is.

(Note: This post has been updated to take into account the 2013 IPCC Physical Science Basis Report.)

The promoters’ arguments – science or red flags?

Basically, the scientific argument is this: greenhouse gases, released primarily by human activities, are causing the earth to accumulate heat energy; this, among other consequences, will result in drastic changes in global climate. In technical terms, it’s known as anthropogenic climate change.

There’s a long history to this argument. It starts in 1896 with the Swedish scientist Arrhenius, who worked out that adding carbon dioxide to the atmosphere would result in global warming. About the middle of the 20th century, scientists started getting concerned about the possible dangers this posed and the number of scientific studies on the topic started to build up. At that early stage, there was no consensus though, because some scientists thought that the earth could be heading into a cooling phase. By the 1970’s, it was becoming obvious that the earth was indeed on a warming trend. Scientists started to look for evidence to show what was causing the warming and began making projections about the consequences. There are now thousands of scientific papers examining the phenomenon. (For more detail, see the excellent histories by John Mason at Skeptical Science or Spencer Weart, here.)

As long as you stick to the peer-reviewed scientific literature, red flags are few and far between on this topic. On the contrary, all the hallmarks of science are there: there is a variety of models; they are tested in the real world; there is a coherent body of knowledge that is supported by peer review; but it is acknowledged that there gaps in our knowledge.

Unfortunately, outside the scientific literature, many red flags appear.

The doubter’s arguments – science or red flags?

Those who doubt the scientific argument do so on three main grounds; that the earth is not accumulating heat energy at all (see here); or, that the warming and climate change are caused not by humans, but by natural cycles (see here and here); or, that any change caused by human activities will be minor (see here). None of these positions is supported by the science. Recently, the science writer James Lawrence Powell surveyed the peer-reviewed scientific literature from the last 21 years. He found that there had been 13,950 papers on global warming or climate change published in that period. Only 24 of those papers (that’s 0.17 per cent) attempted to reject the model of human-caused global warming. The details of Powell’s study are here.

Doubters of the scientific position are prolific producers of red flags. The main ones used are cherry picking (see here), conspiracy theories (see here), the Galileo gambit (see here), appeals to authority (see here) and single-study syndrome (see here). Skeptical Science deals with the whole gamut of the doubters’ PRATT list (here).

Being skeptical

On the basis of red flags, I’m extremely skeptical about the doubters’ claims. This doesn’t excuse climate scientists from supplying supporting evidence though – that would be switching the burden of proof. So I need to look for trustworthy sources of evidence supporting the anthropogenic climate change model. At the same time, I should examine any evidence the doubters present to support any scientific models they propose.

There is also plenty evidence that most doubters are operating from ideological bias or financial interests.  Naomi Oreskes and Erik M. Conway give lots of detail in their 2010 book Merchants of Doubt, and there have been many other studies demonstrating the links – see here, here, here and here. A good summary of the tactics used can be found at DeSmogBlog.

The scientific evidence

Until the 1970’s, the evidence for human-induced climate change was not conclusive. Since then, evidence in support of the model has continued to accumulate. It would be impossible for me to look at all of the peer-reviewed papers on this issue, but fortunately I don’t need to.

The most reliable snapshots of the current state of scientific knowledge on any topic are provided by systematic reviews. In the case of climate change, the Intergovernmental Panel on Climate Change (IPCC) conducts massive systematic reviews about every five or six years. Here’s how the IPCC’s own website describes its role:

The Intergovernmental Panel on Climate Change (IPCC) is the leading international body for the assessment of climate change. It was established by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) in 1988 to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and socio-economic impacts. In the same year, the UN General Assembly endorsed the action by WMO and UNEP in jointly establishing the IPCC.

The IPCC is a scientific body. It reviews and assesses the most recent scientific, technical and socio-economic information produced worldwide relevant to the understanding of climate change. It does not conduct any research nor does it monitor climate related data or parameters.

Thousands of scientists from all over the world contribute to the work of the IPCC on a voluntary basis. Review is an essential part of the IPCC process, to ensure an objective and complete assessment of current information. IPCC aims to reflect a range of views and expertise.

I’ve bolded one paragraph in the above quote because the role of the IPCC is frequently misrepresented by doubters. It’s important to realise that the IPCC reviews the work of scientists worldwide – it does no scientific research of its own. Here’s a good overview of how the IPCC operates, and another explaining how its reports are written..

The IPCC is not perfect, but it is by far the most trustworthy body to consult for reliable information about climate change. Its latest reports are due in 2013-14, and the evaluation of the scientific evidence is already available in the report titled “The Physical Science Basis“. From that report, here are the “headline statements” that are most relevant to the question we are checking on:

• 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.

• Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850 (see Figure SPM.1). In the Northern Hemisphere, 1983–2012 was likely the warmest 30-year period of the last 1400 years (medium confidence).

• Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010 (see Figure SPM.3), and it likely warmed between the 1870s and 1971.

• The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years. CO2 concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.

• Total radiative forcing is positive, and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750.

• Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.

• 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. This evidence for human influence has grown since AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

• Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.

• Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond . Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2.

For some hints on how to approach reading and understanding IPCC reports, see this article at The Conversation.

There is  a very good, not-too-technical summary of the report by Steve Easterbrook here, and the IPCC itself has produced a video summarizing the latest ‘Physical Science Basis’ report:

The small number of scientific papers that doubt human-induced climate change is taken into account by the IPCC reviewing process. As we can see from the IPCC findings, none of these papers has presented evidence that is compelling enough to overturn, or even significantly effect, the accepted model.

(Update 2013/05/16: For those who are reluctant to accept the IPCC interpretation, here are some independent reviews:

 The scientific consensus on climate change
(Naomi Oreskes, 2004)

The scientific consensus on climate change: how do we know we’re not wrong?
(Naomi Oreskes, 2007)

Examining the scientific consensus on climate change
(Peter Doran & Maggie Zimmerman, 2009)

Expert credibility in climate change
(William Anderegg, James Prall, Jacob Harold & Stephen Schneider, 2010)

Quantifying the consensus on anthropogenic global warming in the scientific literature
(Cook et al, 2013))

DIY evidence

It’s tempting to make your own judgement about climate change from your own experience. To many of us, it seems that there have been obvious shifts in local climate since our younger days. But it’s dangerous to draw conclusions from our own limited knowledge. Humans are notoriously bad at making correct statistical judgments from personal experiences. Only long-term observations from all over the globe can give valid data about climate change.

Conclusion

It’s clear to me that the scientific evidence leaves no room to doubt. The anthropogenic climate change model is robust. This does not mean that the model is perfect nor that there is nothing more to be discovered. There are uncertainties, and the IPCC reports go to great lengths to describe what those uncertainties are. Doubters like to exploit the uncertainties in attempts to make the findings appear shaky, but this tactic has no traction within the scientific community. Never has a branch of science been scrutinised as closely as climate science, yet its conclusions remain solid. The science is clear: multiple independent lines of evidence show that we are causing climate change – and it’s serious!

Update 2013/01/04: Climate scientist Katharine Hayhoe has an article on this very topic here. It’s very clear and easy to follow.

Update 2013/05/13: Here’s Peter Hadfield’s excellent video The evidence for climate change WITHOUT computer models or the IPCC. It’s an easy-to-follow summary of the long-standing evidence which shows that atmospheric carbon dioxide has always influenced global temperature.

Update 2013/11/11: Skeptical Science now has an excellent Climate Science History that you can explore interactively.

Update 2013/12/21: If you find even the IPCC Summary for Policymakers is too technical, oceanographer Greg Johnson has produced his own summary in haiku form, accompanied by watercolours.

Update 2014/03/01: The Royal Society and the US National Academy of Sciences have released an extremely readable summary of the state of climate science here: Climate Change – Evidence and Causes.

Update 2014/03/19: The American Association for the Advancement of Science (AAAS) has launched a new initiative to get the scientific message on climate change across to those who are still doubtful: What We Know.

Further reading

Jury in on climate change, so stop using arguments of convenience and listen to experts by Brian Schmidt, astrophysicist and Nobel laureate.

What’s Really Warming the World? by Eric Roston and Blacki Migliozzi at Bloomberg

The Tropical Cyclone Inigo approaching northwest Australia photo is by Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC (http://visibleearth.nasa.gov/view_rec.php?id=5161) [Public domain], via Wikimedia Commons

This is one of ScienceOrNot’s Let’s check the science series.

This article reviewed and updated 2013/09/27