Opinionista Ivo Vegter 6 July 2015

Bloomberg global warming chart proves nothing

Bloomberg recently ran a pretty animated chart that ostensibly clearly shows global warming is caused by human activity. But it makes almost every mistake in the book on graphing statistics. You’d swear they were trying to mislead people.

Bloomberg sustainability editor Eric Roston and ‘data visualisation developer’ Blacki Migliozzi joined forces to create a chart about global warming. It is very pretty, and would look convincing to any layman, and even many experts.

What is really warming the world?” it asks. “Climate deniers blame natural factors; Nasa data proves otherwise.”

If you take the chart at face value, this conclusion seems pretty clear. But let’s see why we shouldn’t stake our future on a nifty bit of graphic design in the media.

Before we do so, note that questioning the usefulness of a chart, or the validity of its conclusions, does not imply that one must take the diametrically opposite view. People make terrible charts about things with which I agree all the time.

Note, too, that most climate change sceptics are not “deniers” of anything. None deny carbon dioxide is a greenhouse gas. Few deny that human activity has influenced the level of carbon dioxide in the air. Most accept that this may have played a role in temperature variations, especially since 1950. They might differ about how wise it is to base expensive policy decisions on the rather shaky predictions of computer models, how dangerous climate change is likely to become, or whether preventive action or adaptation is the best way forward, but they do not deny the basic science.

Some sceptics have taken to calling themselves ‘lukewarmers’ in an effort to avoid the broad, dismissive and insulting epithet “denier”, with which the environmental movement (and some media outlets, like Bloomberg) denounces anyone who doesn’t sing from the same apocalyptic hymn sheet. These people recognise that global warming exists and that human activity plays a role, but believe (correctly) that the question of climate sensitivity to carbon dioxide has not been settled, and it is not an urgent crisis requiring immediate, large-scale changes in the world’s energy use profile.

But back to the graph. Note a few things about it. Besides a vague reference to Nasa’s Goddard Institute for Space Studies, which keeps one of the major US and global temperature records, it doesn’t give any specific source or title. Is this weather station data? A bit of digging and some work trying to match charts to Nasa’s data suggests it is the annual mean line from the Global Land-Ocean Temperature Index.

It looks like a hasty, last minute addition, but there is a y-axis, marked from -2°F to 2°F (1.1°C). It has neither major nor minor ticks, and no gridlines to read values off the graph. There is no x-axis, although the label ‘1880-1910 average’ (an unusual average against which to benchmark climate data) indicates where the zero might be. There are no labels or vertical gridlines between 1880 and 2014, either. We’re clearly not supposed to gather much helpful information from this chart, other than that the long-term trend is rising.

Then they show graphs of the Earth’s orbital variations, total energy output by the sun, and the effect of volcanoes. Note that the y-axis does not change. Presumably, there is some reason to believe that one or more of these factors could cause a warm period in medieval times and a “little ice age” from the 16th to the 19th centuries, but stopped influencing the climate significantly in 1880. Speaking of which, isn’t it misleading to plot a trend line starting at a time when temperatures were particularly cool?

Then we get the impact of deforestation. Last I heard, razing forests was a bad thing that contributes to global warming, but apparently not. The world seems to be cooler without trees, because the albedo effect means darker forests absorb more heat from the sun than lighter farmlands. Fair enough, no need to worry about the forests, then.

Ozone doesn’t make much difference, they say, and ‘aerosols’, or particulate pollution, works like an umbrella, to provide a significant cooling effect.

Even if we assume that the contributions to temperatures from these five factors – orbit, sun, volcanoes, forests, ozone and aerosols – are accurately reflected in this chart, it over-simplifies matters greatly.

It ignores, for example, the inconvenient truth (Al Gore called it “complicated”) that atmospheric carbon dioxide concentrations appear to lag temperature changes over the last 420,000 years. That is, changing temperatures caused carbon dioxide levels to rise, which then may or may not have had an amplifying (positive feedback) effect.


More importantly, in listing the factors that are claimed might impact on the climate, the chart leaves out the largest sources of uncertainty in climate prediction altogether.

Cloud formation is notoriously hard to model, and climate models only have highly simplified placeholders for the many effects of clouds on climate. “Clouds are the largest source of uncertainty in quantifying climate feedbacks and sensitivity,” according to Joel Norris, assistant professor of climate and atmospheric sciences at the Scripps Institution of Oceanography.

Second to clouds is another very large part of the planet: the oceans. The oceans play a complex role, buffering climate responses to greenhouse gases, and providing a range of potential feedbacks, both positive and negative. Nobody could draw a neat line showing the effect of the ocean on temperatures, for example.

Charts such as these make matters appear simple, but none of this uncertainty makes it to the climate models upon which charts such as these rely, however. That systems as large and powerful as the oceans and clouds are not adequately included in climate models, is a reason to be very wary of accepting the predictions of those models.

Finally, we get to the pièce de résistance: the greenhouse gas. As you probably guessed, it is a fairly neat fit to rising temperatures. It is tempting to raise a correlation/causation argument here. Just because carbon dioxide concentrations rose at the same time as global temperatures, does not mean one caused the other.

But in this case, the criticism would not be valid. Remember, the y-axis has still not changed. It is still a temperature range. This is not an omission. It means this is not a graph of greenhouse gas concentrations. This is a graph of the effect that greenhouse gases are supposed to have on global temperatures.

Now think about what that means. It says, “assuming that greenhouse gases have this much effect on temperatures, then greenhouse gases have this much effect on temperatures”. If that sounds circular, that’s because it is circular. It affirms the consequent. Therefore, contrary to Bloomberg’s claim, the graph does not “prove” anything. It merely illustrates an educated guess. It may be correct, but it has no persuasive power.

Climate sensitivity to carbon dioxide concentrations is still an open question. The IPCC reports say it falls anywhere between 1.5°C and 4.5°C per W/m² of forcing. An upper bound three times as high as a lower bound is an uncomfortably wide range of uncertainty. The range has not narrowed over the years, despite considerable research going into trying to pin it down more accurately, so we have better assumptions to feed into computer models that create different future scenarios. (It is more complicated than this. Here’s a great post on alternative scenarios in the IPCC reports, and the tendency to pick the high one to call “business as usual”.)

Bloomberg’s chart does not disclose even its most basic assumptions. They could be showing the low sensitivity scenario (yeah, right). It could be somewhere in the middle of the range. Or they might have assumed a worst-case scenario, to fit their conclusion that, “The only real question is: What are we going to do about it?”

If a journalist can’t think of more than one question about climate change, one wonders whether they’ve made the right career choice. DM



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