What Will Happen As The Warming Continues

Eventually, global warming may get to the point where the present distribution ranges of species of plants and animals are left far behind the areas that they could potentially live in. For example, the potential range of sugar maple (Acer saccharum) might shift hundreds of kilometers to the north, way up into Canada towards the edges of the Hudson Bay (Figure 3.9). When this situation of a much warmer climate arises, the new vegetation zones and communities won't just snap into place overnight. Seeds of plants will have to physically spread over the landscape, across hundreds of kilometers.

From what we can sec of present-day plant distributions, many species will be left growing in temperatures that arc too warm for them, at least in the lower-latitude parts of their distribution ranges. Sugar maple, for example, extends down to Tennessee and if the warming causes climates to shift north it may be left out-of-sorts in the south of its range. If the southern range limits of northern temperate species

Figure 3.9. Sugar maple extends from southeastern Canada to the south-central USA (a). By 2090, sugar maple may he able to grow way up to the southern Hudson Bay (lighter gray area on map). Meanwhile its natural range in the southern USA will tend to be lost as temperatures there become too warm for it (b). Source: Redrawn from IPCC'.

Figure 3.9. Sugar maple extends from southeastern Canada to the south-central USA (a). By 2090, sugar maple may he able to grow way up to the southern Hudson Bay (lighter gray area on map). Meanwhile its natural range in the southern USA will tend to be lost as temperatures there become too warm for it (b). Source: Redrawn from IPCC'.

contract faster than they can expand at their northern limits, they may ultimately go extinct.

How long will it take for the plant communities of the greenhouse world to take shape? Will it be decades, centuries, millennia; or will species just never manage to shift themselves this far? And what will happen to the communities and vegetation types that exist in each place now, when climate warms. For example, will the trees in our forests just die, before others can spread north and replace them?

Questions such as these are very hard to answer, but there are some clues from the past, when warming events—of comparable speed and magnitude to that which we are anticipating over the next century—actually occurred (see above). Such events were associated with the immensely unstable climates of the last 2.4 million years. There were repeated sudden warming and cooling stages, apparently taking only-decades in many cases. For instance, around 11.500 years ago at the end of the cold phase known as the Younger Dryas, a very sudden warming event around the North Atlantic was largely completed in 75 years. This sudden jump is comparable in size and speed with the projected "greenhouse effect" warming over the next 100 years.

From the cvidcncc of responses to past sudden climate changes, it looks like vegetation will remain out of equilibrium with climate for hundreds and indeed thousands of years following the onset of greenhouse cilect warming. For example, in Britain, after a similar sudden warming event 14,500 years ago, vegetation remained out of balance with climate for hundreds of years (above). Trees seem to have been unable to spread north into Britain fast enough to exploit a warm climate which would have suited them, and the landscape remained covercd in a sort of meadow vegetation. We might find similarly strange situations arising in a future greenhouse world over the next several centuries: vegetation types that no longer match the climate, without warm-climate plants having spread in from the south to take their place. When warmer-climate plants do eventually start to arrive, particular species may start to dominate out of all proportion, just as the hazel tree did in England after the warming event 11,500 years ago (above).

We can estimate how fast trees spread after past warming events, from the time delay between them turning up in the pollen record at each lake and then the next ones slightly farther north. For Europe, when trees spread north after sudden warming events, they moved at peak rates of between 0.02 and 2 km a year, depending on the species. In North America the rates of movement were rather slower, between 0.08 and 0.4 km per year. Going simply by these past figures for migration rates, it seems that the geographical ranges of a few species in Europe might almost be able to keep track within a moderate greenhouse warming scenario, where climate moves north at about 5 km a year. They would perhaps show a migration lag of a few decades. The future rate of warming may be expected to vary with latitude, according to climate model predictions, so the rate at which tree species' ranges will need to shift to keep step with climate warming will be greatest at the more northerly latitudes. If the reported migration rates from the past are representative of the northern coniferous and temperate zones of the world, it appears that at all latitudes most tree species would be left far behind, but might catch up on a timescale of centuries or millennia if the warming stabilized.

However, just relying on reported migration rates of trees in the pollen record is a very simplistic way of trying to forecast their future responses to climate change. It is difficult to figure out all the factors that could have affected the rate of movement of trees. In some areas, such as Europe, there were no pre-existing forests in place before the warming, and this probably allowed trees to move faster than they would through the now-forested landscapes. However, nowadays humans often harvest trees from the woods, even clear-cutting whole swathes of forest. Such open areas may provide an ideal opportunity for migrating trees to establish themselves.

In North America, there was already forest covering the eastern USA at the time of the sudden warming 11,500 years ago, but its species composition altered in response to the change in climate. The pollen record from lakes shows that many different tree species spread north, but it generally took between several hundred and several thousand years for them to reach their final limits under the new warmer climate (above). It is likely that, if left to themselves, forests in the mid-latitudes will take a similar period of time to adjust to greenhouse effect warming.

What about areas that were already forested with cooler-climate species of trees before the warming event? Did those trees already in place die in response to the change in climate? Reassuringly, there is no evidence that the rapid warming event at the end of the last ice age in North America was associated with any sudden death of the forests. It seems that the trees already present at the time were tolerant enough of warmer temperatures to survive. Where they disappeared from the forests it seems to have been a gradual process over hundreds of years brought about by competition from other trees that moved in from the south, allowing them the chances to spread their ranges northwards. This makes it likely at least that the forests we sec in the present world will not all die when the climate suddenly warms by several degrees.

However, wc do not really know just how much warming will occur, especially if some of the feedbacks mentioned in the later chapters in this book start to kick in. Eventually, the temperature rise might start to exceed what is survivable. The temperature increases are likely to be particularly drastic in the high latitudes, where various positive feedbacks (see Chapters 5 and 6) will tend to amplify the greenhouse warming. At least one major tree species in Siberia the Siberian larch (Larix Siberian)—seems unable to cope with mild winters regardless of competition from other species and will simply die in place. If it is grown in the mild climates of western Europe, Siberian larch thrives for about 25 years and then suddenly dies, apparently unable to defend itself against attack by fungi in its environment. Whether winters in Siberia will ever become as mild as they now are in western Europe is a moot point, but it does show that there are limits to what cold climate plants can tolerate, beyond which they will simply die. For all we know, other plants from the north might turn out to be even less tolerant of warmth than Siberian larch is.

3.7.1 Movement of biomes under greenhouse effect » arming

The predictions of GCMs coupled to vegetation schemes provide some clucs to what the final distribution of vegetation types in the greenhouse world might look like. Warming of several degrees C is enough to push the ranges of northern temperate trees hundreds of kilometers polewards beyond their present limits. At the same time, range limits in the south arc likely to contract as well (although the picture from the last glacial suggests that this may be a slow process, dependent on other warmer-climate competitors moving northwards to out-compete them).

Movement of many different temperature-limited biomes outwards from the equator seems likely. Some areas that now have temperate climates with frosts are predicted to become tropical. For example, in a moderate warming scenario, by 2100 tropical rainforest is predicted to be the "right" vegetation type for southern Louisiana. However, even if they are expanding at the edges, the core areas of tropical rainforest that exist at present might start to suffer under global warming. A model study by Peter Cox and colleagues suggested that, as Atlantic temperatures warm due to the greenhouse effect, the Amazon rainforest will experience severe droughts. As if to prove this point, a year after this study was published, the Amazon region suffered an unprecedented drought associated with a sudden warming in the equatorial Atlantic.

In the mid-latitudes of the USA. one study using various climate model scenarios by Bachclct and colleagues suggested that with a certain moderate amount of warming there will be a net increase in forest, spreading out over desert and grassland areas as a result of increased rainfall. But they also suggested that if the temperature keeps on increasing above a certain limit, the climate will get drier overall and forest will retreat. Something that complicates many of these modeled future scenarios is that they also include a direct effect of increased CO: on the physiology of plants. As we will explore in Chapter 8 of this book, the influence of higher C02 on the growth and water balance of plants is a big uncertainty that adds to the diiliculty of forecasting effects from climate change alone.

The greatest shifts in vegetation arc predicted to be seen in the high latitudes where warming is predicted to be strongest, and where the most dramatic warming is in fact already under way.

Changes in the amount of rainfall and snow, and in the precipitation/evaporation balance, are seen as being more difficult to predict than temperature. Different GCMs come up with very different conclusions for the amount and distribution of change in rainfall based on only slight differences in their assumptions. Overall, it looks like the changes in moisture balance will not be dramatic over the next century as the global climate warms, with perhaps more rainfall giving slightly less arid vegetation overall across the greenhouse world.

The way in which biome-based models divide up the world tends to give the impression that the only changes which occur during warming are at the boundaries between biomes. However, there are major differences in species composition and physical form of vegetation within each biomc, and it is important to remember that we can expect changes in these just as much as at the boundaries.

All the biome-predicting vegetation schemes we have considered here so far are "static": they simply state what vegetation types will be in balance with a changed climate. They do not tackle the problem of how long it will take for the new vegetation to arrive in a new place and then grow to maturity. We know from the history of past change that vegetation can remain out of balance with climate for hundreds or even thousands of years. To get a better idea of the time course of changes in vegetation as the earth warms over the next century, ecologists have come up with dynamic vegetation schemes, which gradually "grow" new vegetation suited to the changed climate produced by a GCM. Dynamic schemes do not just assume that forest can spring up in grassland or desert areas fully grown overnight; they recognize that it will take decades to mature from seedlings. Examples of such schemes are the MAPSS scheme, and the DOLY scheme.

Although they arc likely to give a more realistic simulation of the time course of events as climate warms, these dynamic schemes do not simulate the complex processes of migration of species which will be necessary in order to alter biome distributions. They simply assume that the vegetation of the future is already in place as seedlings, waiting to grow when the climate changes. Yet, as we know from the aftermath of sudden warming events in the earth's recent history, the time taken for migration can cause a major delay in the adjustment of vegetation to climate.

In the modern world, the process of migration could take even longer than it did after ice ages. The distributions of many plant species arc broken up by agricultural landscapes, making it hard for them to move across sterile fields that arc regularly ploughed and sprayed with herbicides. For instance, in western Europe many types of plants that normally only live in forest would somehow have to hop between isolated woods that may be kilometers apart from one another. The problems of migration may be particularly great for species of European and North American wild (lowers known as "ancient woodland species", because they only seem to be found in very old. established fragments of forest, seeming unable to colonize young forest. One example is the bluebell, Hyacinthoides non-scrip/a, which forms a beautiful blue carpet in English and Welsh woodlands in the spring. It is not clear whether such spccics would ever be able to migrate in response to climatc change under present circumstanccs, given the extra handicap that they suffer due to their restrictive requirements.

However, it is possible that humans can come to the rescue, helping many wild plants to overcome what is in the first place a human-made problem. In the northern mid-latitudes, w hich arc so intensively farmed, deliberate planting of species north of their previous range could allow them to exploit the warmer climate, and make up for loss of range at their southern boundaries. It may require a concerted mass movement of volunteers to plant young trees and flowers farther north in their new potential climatc range. However, it is also important to remember that many spccics of trees and shrubs are already planted well outside their natural ranges in parks, gardens and forest plantations. Beyond their ranges they may exist as poorly-performing and poorly-reproducing individuals, unable to compete with the wild species around them in the current climate. Yet, as climatc warms they may conic into their own and form a natural part of the vegetation. In effect, part of the flora of the future greenhouse world may already be in place, waiting for the warming to happen.

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How To Survive The End Of The World

How To Survive The End Of The World

Preparing for Armageddon, Natural Disasters, Nuclear Strikes, the Zombie Apocalypse, and Every Other Threat to Human Life on Earth. Most of us have thought about how we would handle various types of scenarios that could signal the end of the world. There are plenty of movies on the subject, psychological papers, and even survivalists that are part of reality TV shows. Perhaps you have had dreams about being one of the few left and what you would do in order to survive.

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Responses

  • fiori
    What will happen to tropical rainforests if global warming continues?
    6 months ago
  • leighton
    What will happen if climate change does not stop in tropical rainforests?
    3 months ago
  • adelchi
    What happens when to the tropical rainforst with the earth warming up?
    19 days ago

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