See
Read All About It! Heat Dries Things Up! by Patrick J. Michaels and Paul C. "Chip" Knappenberger of Cato.
"No one doubts that much of the West, especially California, has been
very droughty since the turn of the century, and that heat and drought
are highly correlated. So it seemed surprising that it was big news last
week that forest fires, which require dry fuel, are on the increase out
there.
University of Idaho’s John Abatzoglou and Columbia’s A. Park Williams
used a large family of climate models to calculate various indices of
western aridity (they used eight different measures), which were then
related to the burned-out area every year. About half of the increase
since the mid-1980s was related to climate-modelled warming. The other
half, they say, was from other causes, including natural variability.
The authors also note that some forest management practices may be
contributing to the increasing burn.
The notion that this much drying is caused by dreaded global warming is what made the papers.
Should we use models that can’t even get close to the real-world
evolution of lower atmospheric temperatures in recent decades to
determine how much climate change is human-caused? That’s what they
did—assuming only warming that was not modelled was “natural.”
To say the least, that’s a heavy logical lift when it is so clear that
the models are predicting far too much warming in the lower layers.
It is all too human to not let some else’s work get in the way of
your confirmation bias. So there’s no mention of another explanation for
why it’s so hot and dry there. Writing in the same journal that the
fire work was published in, the Proceedings of the National Academy of Sciences (PNAS), two other western researchers, James Johnstone and Nathan Mantua, demonstrated that virtually all
of the temperature changes in California and the West are related to
changes in atmospheric pressure patterns that occur with or without
global warming. That was first published in 2014, but there is no
reference to it whatsoever in the fire paper.
Nor is there any reference to the most comprehensive study of western
fires—some 33,000 of them—by Argentina’s Thomas Kitzberger showing that
for centuries the distribution and frequency of western fires is
related to well-known atmospheric patterns over both the North Pacific
and North Atlantic, not global warming. It too was published in PNAS, in
2007.
But we digress. Aridity is largely driven by temperature (warmth) and
precipitation. Unfortunately, only two of their eight measures of
dryness are very sensitive to rainfall variability.
Climate models have pretty much no skill in estimating precipitation.
But they do predict warming, and western (particularly California and
Arizona) temperatures are higher than they were. So, absent any
precipitation data, they are guaranteed to paint a drying picture and therefore an increase in fire extent.
The six aridity indicators that are not particularly influenced by
precipitation instead are primarily temperature-driven. Not
surprisingly, these show much greater increases in aridity than the
other two.
Here’s an example from the heavily forested northwest states of
Idaho, Washington and Oregon. One of the aridity indicators is the
Palmer Drought Severity Index (PDSI), an old warhorse that has been used
to assess long-term moisture status since it was first published in
1965 by Wayne Palmer, a scientist at the (then) U.S. Weather Bureau.
Our first figure shows the relationship between observed summer
temperatures and the PDSI. Note that the more negative the PDSI is, the
more droughty it is, and vice-versa for wetness.
Figure 1. Scatterplot
of summer Palmer Drought Severity Index values vs. summer temperature in
the U.S. Northwest climate region, 1895-2016 (data from National
Centers for Environmental Education, http://www.ncdc.noaa.gov/cag/).
That’s the variable that the models as consistent with in their
projections, and it explains about 20 per cent of the summer-to-summer
behavior of the PSDI since 1895.
What about the other component—precipitation—in which model predictions are all over the map? We show that in Figure 2.
Figure
2. Scatterplot of summer Palmer Drought Severity Index values vs.
summer precipitation in the U.S. Northwest climate region, 1895-2016
(data from National Centers for Environmental Education, http://www.ncdc.noaa.gov/cag/).
In this area, about twice as much of much of the PDSI behavior is predicted by rainfall than by temperature.
A different situation obtains as one moves further south, with
temperature more important because it doesn’t rain very much in the
period we are looking at here (summer). In droughty California any
significant summer rain aside from occasional mountain thunderstorms is
usually front-page news. If you average across the entire area, the
contributions of the two predictors are pretty much equivalent.
If dryness is driven both by temperature and precipitation, and we
can’t predict the latter, then climate models guarantee a positive
relationship between climate change and fire in a warming world. This is
exactly what this paper shows. Is such a prescribed result actually
news?"
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