It is an obvious fact that all vegetal species dont thrive everywhere : a palm tree in Norway or an oak tree in the middle of the Sahara won't be very pleased there. Stating this truism is actually a way to say that vegetal - and animal - species that constitute ecosystems are adapted to the local climatic conditions, and might often bear small changes but not survive to big ones.

The main point concerning vegetation is that the amount of available water determines much more the possibility to find a given plant than the average temperature, the threshold regarding the latter being often frost. Hence if precipitations change a little, the vegetation type changes quickly, when if temperatures change a little, with constant precipitations, vegetation types won't necessarily be drastically modified. Said differently, it means that temperature variations are not necessarily dramatic for the vegetation, as long as they remain limited to a couple of degrees, when changes in precipitations can very rapidly be.

We will then have to do with a "generic" exam of the possible consequences of climate change on vegetation, complicated by the fact that enriching the atmosphere in CO2 is something rather positive for vegetation (but it is of course not possible to dissociate the biological benefit of enriching the air in CO2 from the adverse climatic evolutions that might result from the same CO2 !). So how does a climate change can affect plants, and, consequently, ecosystems ?

A climate change can move suitable zones too quickly for the vegetation to follow :

A suitable zone corresponds to a region where it is "pleasant to live" for a plant . If it moves slowly it is not very serious : the population of plnats moves also and survives (see little figure below). If the suitable zone moves too quickly, the population disappears because it doesn't have the time to reproduce and migrate before the conditions cease to bo adapted.

depending on the species, the maximal "migration speeds" vary from 4 to 200 km per century. This speed is lower for plants that are mature late and have low mobility seeds (that therefore can't go very far from the favourable zone of the moment) ; oak trees (sexual maturity at 50 years, heavy seeds and few animals that transport the acorns) represent a typical species with a slow migration speed.

Well according to the IPCC a global warming of 3° C will generate, for the temperate zones, a polewards displacement of the favourable zones of 500 km or so. 3° C in a century - median evolution in the bracket of the present predictions going from 1 to 6°C - therefore generates a displacement speed by far superior to the 200 km at most indicated above. In addition, it is probable that continents, that don't have the thermal inertia of oceans, will undergo superior temperature increases. A large number of natural species, including trees, might therefore vanish in case of a brutal modification of the climate, because where the suitable zone used to be conditions became too hostile. An example of such a change is given below for two species in France (beech and spruce).

NB : a "suitable zone" doesn't mean that a given species will be necessarily found somewhere, but only that it can be found. For the tree taken in example, if people chop down everything, if a pest decimates a whole population, etc, there will not be any trees left in a place where they neverthelss could be !

Potential distribution of beeches in 2000 (see colour meaning below). Source : Modélisation et cartographie de l'aire climatique potentielle des grandes essences forestières françaises, Badeau et al., june 2004.

Simulated distribution of beeches in 2100, with the B1 scenario (constant emissions over the 21st century, more or less). Numerous zones of the country would become too hostile for this tree (beeches presently living in these places will thus die). This simulation is of course not a forecast : impossible to take into account the effect of pests, of competition between species, and of course of the action of man (destructive or not).

Colour code used depending of the probability to have beeches in a given place.

Potential distribution of spruce in 2000 (see colour meaning below). Source : Modélisation et cartographie de l'aire climatique potentielle des grandes essences forestières françaises, Badeau et al., june 2004.

Simulated distribution of spruce in 2100, with the B1 scenario (constant emissions over the 21st century, more or less). The same remarks than for beeches apply.

Colour code used depending of the probability to have spruce in a given place.

If the favourable zone moves, it means that the zone previously favourable has ceased to be. What might have happened ?

the weather might have become too dry (8.000 years ago part of the Sahara was covered with an abundant vegetation, hence proving that a local weather can become much dryer). A "dryer climate" can actually result from a mixture of less precipitations, precipitations concentrated on a restricted number of heavy rain, and more intense evaporation, both the latter leading to a dryer soil.

Multimodel ensemble of the regional precipitation change for the 2080-2099 period compared to the average over 1980-1999, with the A1B emission scenario (emissions roughly double over the 21st century). The difference is scaled in mm of rain per day, knowing that 0,5 mm/day ≈ 180 mm/year, that is one fourth to one fith of the annual total for most mid-latitudes. Regions are stippled where at least 80% of models agree on the sign of the mean change. From IPCC, 4th assessment report, 2007

Multimodel ensemble of the regional evaporation change for the 2080-2099 period compared to the average over 1980-1999, with the A1B emission scenario (emissions roughly double over the 21st century). The difference is scaled in "equivalent mm of rain per day", knowing that 0,5 mm/day ≈ 180 mm/year, that is one fourth to one fith of the annual total for most mid-latitudes. Regions are stippled where at least 80% of models agree on the sign of the mean change. Be careful ! green and blue zones mean that evaporation increases (and thus means a drying if everything else remains unchanged). From IPCC, 4th assessment report, 2007

Multimodel ensemble of the regional soil moisture change for the 2080-2099 period compared to the average over 1980-1999, with the A1B emission scenario (emissions roughly double over the 21st century). Regions are stippled where at least 80% of models agree on the sign of the mean change. A dryer soil is generally not good news for the local biomass. Note that even some places where precipitations increase might end up with a dryer soil (part of Northern Europe, Canada, part of Siberia). From IPCC, 4th assessment report, 2007

 

the weather might have become too hot : in the mid-latitudes, vernalization (the fact to be exposed to low temperatures during the winter) is indispensable to many plants to be able to produce fruits (and seeds, of course), and winter is also very useful to many plants to get rid of pests, killed by the cold, and that proliferate otherwise.

the weather might have become too rich in extreme phenomena, that physically destruct some plants, can degrade the soil, or bring elements that prevent vegetation from growing properly, etc.

A climate change can increase the competition of other species. For example, conditions might become particularly favourables for micro-organisms causing diseases (fungii, microbes...) to plants or animals, or for destructive insects (fancy locust clouds in the fields near Dusseldorf ?),

A climate change can make the context propitious to human damages, particularly the setting of fires which is difficult to avoid in case of drought (and let's not forget that most of the earth is populated !),

A climate change can more genrally destabilize the balance of an ecosystem, what can lower its biodiversity or, in the worst cases, kill it. During the ice ages, with a world average temperature of "just" 5 °C lower than today, the ecosystems that we know in Europe were not present in the middle latitudes. It is difficult to say what would remain of present ecosystems on a planet that would "warm" by 5 or 6 °C in a century, but in many regions of the world it might mean a big shock.

Agriculture is a particular type of land use, hence everything mentionned above applies, but in this precise case man can attenuate certain climatic disorders (it is possible to partly remediate to drought by irrigation, at least as long as it rains not too far from the place to irrigate, to cold by greenhouse farming, etc).

we have to take something else into account : for a vast mojority of crops, only part of the plant is of high intesrest : fruits or seeds (cereals, fruits, even tomatoes or eggplants !), leaves (cabbage, salad...), tige (sugar cane, or... trees), roots (bettroots, carrots...), tubercules (potatoes), flowers (cauliflowers, artichokes...), etc. This means that tthe question of the adaptation must be cut in two :

how the whole plant reacts to the climate change induced,

how this reactions is "spread" among tige, leaves, fruits, roots, etc, and whether the overall result for the part which is really desired can be negative while photosynthesis nevertheless increased. Fort example enhanced leave growth can lead to a decrease of fruit production.

And at last a plant reacts both to a climate change and a CO2 concentration change. Taking all the interactions into account is not that simple ! Agronomists, however, have tried to figure out what could happen, which is summarized below for the major cereals.

But a number of factors are not - or poorly - accounted for in simulations :

the possible spreading of new or existing pests (insects, fungii, viruses...),

competition between species, particularly with adventices (that we call "weeds"when gardening),

the qualitative change of a given part of the plant (sugar content, size of the grain, etc).

a possible increase of climate variability.

These results therefore remain indicative, and here as always the main risk factors lie in what it is not possible to evaluate today !

Agriculture has an additional weakness compared to "natural" ecosystems : in developped countries, it has become highly specialized, which means that, based on a small number or varieties, it has high performances in a narrow gap of climatic conditions, but out of this gap the output decreases quickly. Agriculture is therefore particularly vulnerable in case of a strong variability increase, or in the case of the apparition of new pests well suited to a given plant.

As seen above, climate change won't be restricted to a temperature change, and precipitations will also change. This can harm aquatic ecosystems.

These ecosystems and the fish populations might suffer from a succession happening too rapidly of high and low waters, or of excessive water temperature increases,

the changes in freshwater ecosystems might in return have an impact on the avaiability of freshwater.

It seems pretty unlikely, given what a couple degrees on the average temperature of the earth represent, that 90% of ecosystems would "skip through" with no harm in case of a significant temperature increase. This being said, trying to be precise on the localization and the nature of possible major changes still remains a risky business....

To know more (for confirmed readers) :

- download (PDF format), the summary for policymakers of the working group 2 of the IPCC : The regional impacts of Climate Change, an assment of vulnerability.