Transcript Climate Change – effects on plant ecology

Global Climate Change and its Effects on Plant Ecology
The climate change forecast to be occurring and to massively
change climate, sea level, the intensity of storms, the amount
and distribution of precipitation will obviously affect plants.
The question is how and how much?
A related question: how much is anthropogenic in origin?
How much results from increasing amounts of greenhouse
gasses and how much from other human activities (e.g. land
use, urbanization)?
We can’t develop definitive answers to all those questions, but
we can examine some of the evidence.
The global carbon cycle – pools (in units of 1015g C) and
fluxes (in units of 1015g C/yr). Acronyms:
DOC – dissolved organic carbon
DIC – dissolved inorganic carbon
GPP – gross primary production
Rp – plant respiration
The previous diagram considered only natural, active pools
and fluxes. Thus it did not consider combustion of fossil fuels
nor the estimated size of the total fossil fuel pool. Nor did it
consider the amount of carbon stored in sedimentary rock.
The largest active pool is the carbon dissolved in the oceans,
and most of that is in deep waters. That pool is exchanged
only slowly; estimated turnover time is 350 years. The amount
in surface water is only a small fraction of that total, but it6 is
exchanged much faster (average turnover is estimated as 11
years).
The oceans currently remain a sink for carbon, i.e. more is
dissolved in the oceans each year than is released back into
the atmosphere. One of the key questions is how much longer
the oceans will remain a sink?
If the oceans cease acting as a sink, then the atmospheric
concentration of CO2 will begin to rise much more rapidly.
Even with the moderating effect of solution in the oceans,
CO2 has increased in the atmosphere, from ~280ppm at the
outset of the Industrial Revolution (when fossil fuel
combustion began to increase rapidly) to a current level of
between 370 and 380ppm.
Most estimates suggest that the level could rise to around
600ppm within 50 years unless the Kyoto Accord or an
alternative treaty is reached within a very few years.
The evidence of the history of atmospheric CO2 over the last
1000 years or more is clear. Early portions of that history
come from measurements of atmospheric gasses trapped in
bubbles in Antarctic ice (the Vostok core).
More recent data comes from Greenlandic cores, and most
recently from the Mauna Loa Observatory on Hawaii.
CO2 has direct effects on plant photosynthetic rates.
You might expect that the increasing CO2 would increase net
photosynthesis in C3 plants, but note the compromises that
increasing temperature and photorespiration may have. Water
use efficiency should increase, but increasing temperature in
part may cancel that out.
The data that have been collected thus far seem to bear out
increased net photosynthesis in C3s with increasing
atmospheric CO2, but most are laboratory studies, and
remember that air temperature has not yet increased that
much.
The same logic says that C4 plants should lose much of their
advantage over C3s. Is that borne out?
In a word, NO!
C4 grasses also increase their rates of photosynthesis in
elevated CO2, though less than C3 grasses used in the same
experiments (25% versus 33%). Plants with both
photosynthetic systems grew to greater total biomass (by 33%
and 44% respectively), and both showed greater water use
efficiency.
Is the whole story told by change in CO2 in the atmosphere?
Again the answer is NO!
As you well know, the key to climate change is the greenhouse
effect, and that is due to more than just carbon dioxide. The
other major contributors are: methane (CH4), nitrous oxide
(N2O), ozone (O3), and chlorofluorocarbons.
Methane is released in marshes, from boreal forest
(particularly as it warms), from natural gas leakage, from
decomposition in flooded rice paddies (or from areas flooded
when hydroelectric dams are built and submerged plants
decompose), from decomposition in garbage dumps, and from
ruminant digestion. It is 20x more efficient at infrared
absorption than CO2. Once more, historical values can be
drawn from ice cores and current data from remote sampling
stations:
Nitrous oxide is yet more efficient, absorbing 270x the
infrared energy as a molecule of CO2. The main sources are
microbial decomposition of nitrogen fertilizers and in
production of artificial fibers from organic precursors (nylon
is mentioned in the text). Here is the global increase indicated
by data from Alert, NWT all the way to the South Pole
station:
Ozone has two faces: in the stratosphere (10 – 50 km above
earth’s surface) acts as a greenhouse gas, but also as an
absorber of incoming UV radiation. The latter function is
more important, since UV can be disruptive of intracellular
molecular architecture. In the troposphere (lower atmosphere)
ozone is formed by chemical reactions among nitrogen
oxides, volatile hydrocarbons, and CO under light. That ozone
acts as an irritant to the human lung.
Chlorofluorocarbons are synthetic molecules developed as
stable refrigerants. The dominant forms used have been
CFC11 and CFC12. Treaties (the Montreal Accord) mandated
ending the manufacture and distribution of CFCs, but the very
persistence that was their advantage have meant they remain
in the upper atmosphere, where they act as greenhouse gasses
and react to reduce stratospheric ozone.
Is there clear evidence that there has been global warming?
Emphatically, YES!
This figure compares
temperatures over the
last 125 years to the
long-term average
temperature, all
representing averages
over thousands of
reporting stations:
An aside: impacts of global warming on human populations:
One of the key effects of warming is alteration in the
distributions of disease vectors. Here’s a table of some
diseases that are likely to be affected:
Disease
Vector
Malaria
mosquito
Schisosomiasis snail
Sleeping
Tsetse fly
sickness
Dengue
mosquito
Yellow fever
mosquito
Population
at risk (x106)
2,100
600
50
?
?
Distribution
(sub)tropics
(sub)tropics
African
tropics
tropics
tropics
Malaria has already shown climate-related increase. A
1ºC increase in Rwanda in 1987 led to a 337% increase in
malaria incidence. The reason: the mosquito vector, Aedes
aegypti, was able to move into mountainous areas where it
had never been seen before.
Models of climate change and population distribution suggest
that the predicted 3ºC warming will cause 50 – 80 million new
cases of malaria per year.
Is exposure limited to tropics and subtropics? No. An
outbreak of hantavirus (carried by a mouse vector and spread
in its poop) occurred in the southwestern U.S., killing 27, as a
result of the climate warming and increased rains in the El
Niño event of 1993. Without mosquito control, dengue could
enter Florida today.