Peruvian Glacier’s Growth and Retreat. It's a good example of how news
about climate change could easily be misread as indicating more doubt
than there really is. The headline makes it sound as if the link between
glaciers and temperature is so tenuous that this is the first evidence of it,
and that it has been established for only one glacier. The truth is very
different, even though the headline and article are not wrong once you
understand the context. This post is aimed at helping teachers
and students with the basics, and then use that to parse the news.
Over a century ago, it was known that carbon dioxide impedes the flow
of heat (in the form of infrared light) from the Earth out into space,
while not impeding the flow of heat (mostly in the form of visible
light) from the Sun to the Earth. If not for this natural greenhouse
effect, Earth would be much colder. Teachers can demonstrate quite
directly that carbon dioxide impedes the flow of infrared light, but
many teachers may not have the right equipment. Here's a video
comparing the temperature rise of two bottles, one with elevated
levels of carbon dioxide and the other with standard air. And here's
a nice video using an infrared camera to show quite directly that
infrared light is largely blocked by carbon dioxide.
Around the same time (1896) Svante Arrhenius recognized that humans
were pumping ever more carbon dioxide into the atmosphere, and that
this would lead to warming. But "warming" sounded reasonably
beneficial, especially given Arrhenius's prediction that it would take
place slowly over thousands of years. Arrhenius did not account for
the large increase in population over the ensuing century, nor for the
large increase in per-capita use of fossil fuels (cars, airplanes,
etc). Worldwide, we now emit about 17 times the carbon dioxide emitted
in 1896, so change is coming much faster. And now we know that an
increase in temperature is not as beneficial as it may sound because
it can radically change weather patterns, which imposes large costs on
humans as well as on many species which cannot move and adapt rapidly
enough. Apart from that, Arrhenius deserves kudos for his prescience.
Yet if we heard this prediction in 1896 we would be justified in
expressing some skepticism. Earth's atmosphere and oceans (where most
of the excess heat is deposited) form a complicated system, and the
response of a complicated system to a simple input (more heat) may
well not be a simple result (higher temperature). But healthy
skepticism goes only so far; unless you have a better model, you have
to admit that the best model predicts warming. Just saying "it's a
complicated system" does not give you the right to reject all models.
In this case, you would have to figure out where the extra heat would
go without causing increased temperatures, and you would have to have
some evidence to motivate belief in that model.
Fast forward to 2014. Warming is here, and we've learned a lot about
climate models in the meantime. We did find complications (El Nino,
for one), but the simple model was reasonable in its overall
prediction. More heat does mean a higher temperature.
One way "climate skeptics" (I put the term in quotes because
oil-company funding leads to a kind of "skepticism" different from the
detached sort of skepticism we encourage in science) sow doubt about
this result is to suggest that the warming may be due to natural
cycles. There certainly are natural climate cycles, but rather than
treat them in detail here I want to make a bigger point about how
science works: When a model makes a prediction and the prediction
comes true, we should gain confidence in the model, and we should lose
confidence in models which made contrary predictions. Yes, it's
conceivable that the greenhouse model's prediction came true through
a fluke of natural cycles rather than accurately modeling how nature works
...but how much confidence would you put on that possibility?
A prediction is a powerful thing, so let's note the distinction between
a prediction and a retrodiction (or postdiction), which is when you make a
hypothesis after looking at the data. Using data (rather than laws of
physics or other guiding principles) to generate hypotheses is a fine
thing to do, but because "patterns" can randomly appear in data you
cannot confirm the hypothesis with the same data which generated it;
you must seek out new data. (Admittedly, even scientists sometimes
forget to apply this principle.) Climate skeptics can suggest
alternative causes for the warming after looking at the data, but we
should have much more confidence in the model which actually predicted
the data.
Now, the news: a reconstruction of the timeline of growth and
shrinkage of a Peruvian glacier shows that shrinkage is most highly
correlated with temperature and not with other factors such as
precipitation. You have to get halfway through the article to get the
background:
land ice is melting virtually everywhere on the planet...the pace seems to have accelerated substantially in recent decades as human emissions have begun to overwhelm the natural cycles. In the middle and high latitudes, from Switzerland to Alaska, a half-century of careful glaciology has established that temperature is the main factor controlling the growth and recession of glaciers. But the picture has been murkier in the tropics. There, too, glaciers are retreating, but scientists have had more trouble sorting out exactly why.
So, you may have started reading the article thinking that scientists
understood very little about glaciers if they were just now finding a
"link" between glacier shrinkage and temperature, but you now see
that a lot of important knowledge has already been established.
Newspaper articles are designed to tell you what's new first, so it's not
the writer's fault that this background was buried deep in the article.
Nevertheless, in practice many readers will just read the headline and
skim the first part of the article, thus missing this crucial background.
Teachers and students should be aware of this when reading science news.
But wait, there's more! The article goes on to explain how the details
of tropical glaciers are different from most glaciers (intense
sunlight can vaporize the ice directly, and the sunlight lasts
year-round) but that one group of scientists has studied the matter
and still concluded that temperature is the driving factor in
shrinking tropical glaciers. "But a second group believes that in some
circumstances, at least, a tropical glacier’s long-term fate may
reflect other factors. In particular, these scientists believe big
changes in precipitation can sometimes have more of a role than
temperature." In other words, this is a legitimate scientific dispute, but
it is about the details of a very specific type of glacier and has little or
nothing to do with overall concerns about glaciers (or sea ice)
melting worldwide, much less about the reality of climate change. Yet
someone who wants to sow doubt about climate change can point to this
and say "scientists don't really understand why glaciers melt" and people
who don't read the article carefully may well be snookered by that.
Please make sure you (and, if you are a teacher, your students) don't
get snookered.
My next post discusses two more aspects of the nature of science---uncertainty
and model-independent statements---in the context of climate.
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