In what I THOUGHT would be the last of blogs on climate
change, I’ll again start with the conclusions reached so far:
(1)
there is overwhelming evidence that the planet has been in a warming trend—at
least up until the last 10 years or so;
(2)
warming trends are historically correlated with increasing levels of CO2;
(3)
mathematical models of future global temperatures are now predicting much less
warming than previously thought;
(4)
historically, climate change has been cyclic, with small cycles within larger
cycles;
(5)
how the various natural “forcings” of climate change (such as the Milankovitch
cycle) will interact with increased CO2 and other greenhouse gases
is, of course, unknown.
There are three greenhouse gases of interest to us here (or
at least to me!): water vapor,
methane, and C02. As
mentioned previously, greenhouse gases are gases that trap heat and thus warm
the atmosphere. The earth we know
and love has a temperature that is “just right” because of greenhouse
gases. It is estimated that if our
atmosphere did not contain greenhouse gases, the surface of the planet would be,
on average, about 2F rather than the comfortable 57F it is now.
When water vapor, carbon dioxide, and methane are ranked in
terms of their relative contribution to the greenhouse effect, water vapor and
clouds account for 36-72%, carbon dioxide 9-26%, and methane 4-9%. Further, there is a feedback loop in
which atmospheric water vapor increases as the planet warms and temperatures
increase as water vapor increases, until some equilibrium is reached.
Now, methane and C02 do not have the same global
warming potential because there is less methane than C02 in the
atmosphere. But this is
counterbalanced somewhat by the fact that methane hangs around in the
atmosphere for about 12 times longer than C02. The residence time of a C02
molecule is about 3-4 years, but a methane molecule lasts about 36-48 years.
C02 is the most famous (or infamous) of the
greenhouse gases, and I’ll spend the rest of this blog on it. We’ve seen earlier that it fluctuates
along with global temperatures, going back at least 400,000 years. And, unless you have been living under
a rock, you know that C02 has received most of the attention in
terms of climate change because, at least in theory, humans can slow down or even
reverse the warming of the planet by reducing their C02 output.
At the present moment, there seems to be something of a
disconnect between the increasing C02 levels we are seeing and the
“flat” global warming we’ve had for the last 10 years. This doesn’t mean that
C02 is not influencing warming; rather it just indicates that there is something else going
on that may be “blunting” the effects of C02. Does this mean the models are wrong? Or is there something else that is causing
the planet to cool?
The Fourth Assessment Report of the United Nations
Intergovernmental Panel of Climate Change states that the increased warming
we’ve seen since the mid 1900’s is at least 50% due to increased C02
levels.
And there are numerous potential effects of increasing C02
levels that go well beyond warming.
These include increasing plant growth; decreasing plant growth (?!);
changing species mixtures; changing ecosystems, both natural and artificial
(agriculture); and ocean acidification.
As mentioned above, C02 hangs around in the
atmosphere for a relatively short period of time. What happens to it? It either gets absorbed by plants via photosynthesis or it
gets taken up by the oceans.
The C02 cycle is very interesting, highly
complex, and is essential to an understanding of the role of C02 on
the planet. There are about 800
billion tons of C02 in the atmosphere. Of that 800 billion tons, about 215 billion tons are taken up
by the planet each year, either by plant growth (123 billion tons) or
absorption into the ocean (92 billion tons). 219 billion tons are released back into the atmosphere—60
billion tons are released by plants (through respiration), 90 billion tons are
released back into the atmosphere by the ocean, and 60 billion tons are
released by microbial decomposition.
And finally, humans release 9 billion tons of C02 into the
atmosphere by burning fuel, making cement, and stirring up the soil. So, if you do the math, this means that
there are about 4 billion tons left over, and this accounts for the increasing
levels of C02 in the atmosphere—now about 400 parts per million
(ppm) compared to the estimated 280 ppm of the pre-industrial world.
Now I got the information in the last paragraph from
Wikipedia. Which presumably
represents a collective opinion of scientists around the world. But, these numbers really do beg
credulity—for example, there is no scientific estimation that does not include
a range of estimates that spans, for example, a 95% probability. After all, the 9 billion tons of C02
produced by humans is only 1.1% of the 800
gigatonnes in the atmosphere. That
is a pretty small number to estimate correctly. I mean, you’d think the ocean’s absorption would be estimated
at 92 billion tons plus or minus at least this number.
Further, I looked at a European study published in 2011 that
compared the C02 production of countries around the world, and THOSE
scientists said that the amount of C02 released by humans each year
is 34 billion tons. So what’s to be
believed?
Nevertheless, C02 IS going up according to
measuring stations around the world, and they are pretty concordant. So I don’t think the fact that C02
is increasing is a matter of scientific controversy. Additional evidence that C02 levels are being accurately
measured is that levels of C02 track growing seasons. In other words, atmospheric C02 levels
increase during the winter months when the northern hemisphere is cold (because
of decreased photosynthesis) and decrease during the summer months (because of
increased photosynthesis).
However, there is one curious anomaly: although global emissions of C02
dropped about 1-2% during the recent worldwide recession, there was no
concomitant drop in atmospheric C02 levels as measured by reporting
stations. What’s the deal with
that? Perhaps a 1-2% reduction of
emissions is not enough to impact atmospheric C02 on a global scale. If that is true, it has a lot of
ramifications vis-à-vis the use of economic mitigation for C02 reduction.
To change topics, it is interesting to speculate on what
effect increased C02 will have on the planet other than
warming. We have already alluded
to two of them—plant growth/ecosystem change and ocean acidification.
C02 is an atmospheric “fertilizer” for plant
growth—it has been used for decades in commercial greenhouse operations. Consequently, it is natural to posit
that increased C02 will in turn increase the yield of crop plants,
forests, and pastures. Which can’t
be a bad thing, right?
So, what does the data show? There have been literally
hundreds of studies published on the effect of elevated C02 (EC) on
different species of plants under varying conditions. And the results have been, well, varying. In general, the answer is, yes, elevated
C02 increases crop yields and as well as woody plant growth, which occurs
primarily through stimulation of photosynthesis. Studies show that increasing current C02 levels by
400 ppm to 550 ppm will increase crop yields by 10-20% for C3 plants
(like wheat, rice, soybeans) and 0-10% for C4 plants (such as
corn). Under these conditions, the
above-ground biomass of trees increases from 0% -30%, with younger trees being
most affected and little to no change in mature forests.
However, the results of these controlled studies will
probably not be seen in nature because there are many other factors that could
limit plant growth such as insect pests, lack of nutrients, and insufficient water,
all of which may be augmented by increased temperatures. For example, at 450 ppm C02,
the yield of rain-fed wheat increased along with increased C02 until
the temperatures increased by 0.8oC, but decreased when temperatures
increased by 1.5oC or more.
And additional water was required to offset the increased warming. Recent results with soybeans show
similar results—increasing temperatures may negate the fertilizing effects of elevated
C02. However, results
with tropical rice varieties show consistently higher yields with increased C02
and temperature.
Only one such experiment with desert plants has
been reported, and that was this year (2013). It showed that elevated C02 had no effect on either
above- or below-ground biomass over a 10-year period in the Mojave Desert
(southern California). The authors concluded that this was probably due to low
rainfall.
Further, EC may have very unexpected results, such as, for
example, “undoing” the dwarfing genes of dwarf rice. That’s right, it was shown that EC caused high-yielding
dwarf rice to grow taller and fall over, thus losing the benefit of the
dwarfing genes.
For reasons in addition to the fact that I like northern
Wisconsin and its forests, I’d like to talk about a 2011 report on a long-term
(12-year) elevated C02 study of forest trees in Wisconsin—because I
think there are some important lessons here. Experimental plots of poplar, sugar maple, and birch were
established with different varieties, using varieties of each species that are known
to respond differently to elevated C02. It was found during the LAST three years of the study that total
above-ground biomass production increased 40% in 2006, 14% in 2007, and 25% in
2008. The growth rate of some
varieties of each species increased, and the growth rate of other varieties was
unchanged. This is important
because (1) it is counter to other
studies finding that older trees showed no response to EC, and (2) it indicates
that there is genetic variability with respect to EC response.
The latter point is very important, I believe, because the
extent of genetic variability with respect to elevated C02 is really
unknown at this point in time. However,
if we assume that genetic variability for EC is a general phenomenon (which I
bet is the case because it holds for every other trait that has been examined),
this means that further crop improvement in an EC world is possible. That is, I’d be optimistic that
although present day crops of wheat and soybeans, for example, may have
dampened effects from EC combined with increased temperatures, varieties with
favorable responses to EC and elevated temperature probably can and will be developed.
This also suggests that natural ecosystems will adapt. They
will change for sure—some species will adapt and some will not. Maybe northern
hardwoods will move further north into Canada. Maybe grasslands will move to where forests now reside. Or maybe the opposite. Certainly
species compositions will alter in existing forests. And, overall, the earth may in fact have increased
productivity due to EC. Plant
breeders will just develop new varieties under different conditions in the
future.
And finally, elevated C02 generally increases a
plant’s ability to use water. That
is, EC improves the efficiency of plants to make more plant tissue. This is
because when there is more C02 in
the air, the plants can close down little pores in their leaves (called
stomata) that let C02 in and water out—and still get all the C02
they need, while losing less water through transpiration. This improvement can be quite large—increased
water-use efficiencies of 30% have been observed.
Overall, this does not strike me as being a catastrophic
result of elevated C02.
Another issue is that of “ocean acidification.” Estimates indicate that the surface of
the ocean has already become more acid.
Acidity, as measured by an index called “pH,” has gone from a value of
8.25 in 1751 to 8.14 at present. This might not seem like much, but pH scales go from 0 to 14,
where “7” is neutral. Any value lower
than 7 is “acid”, and anything above 7 is “basic.” So at present, the ocean is basic, but it is becoming more
acid.
The theory is that due to the increase in greenhouse gases
in the atmosphere, lots of C02 (an estimated 30-40% of the C02
released by human activity) is being absorbed by the ocean. And when C02 is dissolved in
water, it makes carbonic acid (H2C03).
The most obvious effect of a more acidic ocean is that the
increased acid dissolves calcium carbonate (CaC03). This is important because shells,
coral, and the exterior structures of many sea animals are made up of CaC03. A good example of an animal whose life
history could be interrupted is the Coccolithophore, a tiny algae or
phytoplankton covered in a thin layer of CaC03. This could have catastrophic consequences for other life forms
because phytoplankton are the ocean’s analogy to land plants: they are the
lowest level of the food chain and serve as food for many animals that either
eat them directly or eat the animals that eat them. Entire food chains could be impacted by anything that
diminishes phytoplankton populations.
So it is obvious that losing CaC03 in the ocean
is a dangerous consequence of acidification. A less obvious result is that as the ocean becomes more
acidic, CaC03 itself degrades into C02, making for even
more C02.
But wait. Oops! It turns out that the above theory does
not hold up with regard to all experimental results or field observations. At least in some coccolithophore
species.
A 2008 study showed that increasing C02 levels in
water INCREASED calcification of at least one coccolithophore species. Further, studies of calcification during
a period 55 million years ago when C02 levels may have been more
than 1000 ppm and temperatures were elevated 11oF over a period of
20,000 years (the “Paleocene-Eocene Thermal Maximum”) showed NO changes in
calcified nanofossils with regard to abundance or species compositions. Nor has there been a decrease in
coccolithophore species from 1800 to modern times—in fact there has been a
slight increase in coccolith total mass.
So apparently C02 fertilization increases photosynthesis
enough to allow phytoplankton to regenerate their shells rapidly enough to
replace any dissolved calcium, and even increase growth.
My point here is NOT to make the case that ocean acidification
does not matter. It does, and
there are many studies showing its negative effects on many organisms. My point
is that different species react to their environments differently, as with the
elevated C02 studies in land plants cited above.
I also want to make the point that elevated C02
and elevated temperature will not result in planetary collapse. It may and probably will lead to
changes in species and ecosystems, but nature seems to be robust in its ability
to evolve, thanks to genetic diversity.
A future world may not look like the one we have now—perhaps no ice
sheets, no glaciers, a rise in sea levels, increased desertification in some
places, as well as displaced or relocated human populations. And, of course, not all species may
survive, at least in their present distributions—the iconic polar bear may give
way to its grizzly cousin.
And new ecosystems may expand—a recent report found that
tundra on Ellesmere Island “came back to life” upon retreat of the glacier that
had covered it with ice for 400 years.
Don’t forget that mammals started their ascendancy during
the “Paleocene-Eocene Thermal Maximum” time period—and be glad for that!
There is more to be said about global warming. There is the issue of a rising sea
level—and of course the various proposals that have been advanced to halt or
reverse global warming.
More next time.
Useful references:
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2004.01224.x/full
