Global Climate Change and its Effects on Plant Ecology

1. 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.

2. 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

3. 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?

4. 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).

5. More recent data comes from Greenlandic cores, and most recently from the Mauna Loa Observatory on Hawaii.

6. 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?

7. 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.

8. 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:

9. 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:

10. 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.

11. 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.

12. 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:

13. 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: DiseaseVector Population Distribution at risk (x106) Malaria mosquito 2,100 (sub)tropics Schisosomiasis snail 600 (sub)tropics Sleeping Tsetse fly 50 African sickness tropics Dengue mosquito ? tropics Yellow fever mosquito ? tropics

14. 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.