Curves of temperature variation (in red) and CO2 variation (blue and green) over the last 800,000 years.
So what does it mean that we have reached a level of 400 ppm of CO2 in the atmosphere? Simply put, temperatures will continue to rise until they reach a new state of equilibrium. Knowing that in the Pliocene the global average temperature was 2 to 3 degrees higher than it is today, this means that even if the release of CO2 into the atmosphere is completely interrupted, temperatures should continue to rise until they reach this equilibrium level. In other words, the famous "maximum" of 2 degrees of warming that Humanity has set itself as a limit not to exceed will probably be reached even if we completely stop consuming hydrocarbons. And yet, on the contrary, the consumption of hydrocarbons has not ceased to increase during the last decades...
Therefore, assuming with honesty that temperatures will not stop rising during the next decades and centuries, I find it interesting to take a look back and examine, from the data of geologists and paleontologists, what our continent was like at that time. If the principle of uniformitarianism developed by James Hutton and Charles Lyell is correct (its application, in any case, is what has allowed geologists and paleontologists to reconstruct the history of our planet), then nothing prevents us from thinking that what we know about the Pliocene can be applied to understand what could happen in the future. In other words, the past is, in some way, a mirror of the future and can help us to understand the great changes we must expect in the future. We will now describe some of the consequences of climate change that the Pliocene study allows us to anticipate.
See level rise
One of the most worrying consequences of climate change, which is often underestimated, is the evolution of sea level. Somehow, we tend to see the large Arctic and Antarctic ice masses as immovable, and we tend to think that the changes affecting those ice masses are very slow. The sudden acceleration of the melting of the Groenlad ice sheet in recent years, that in last years was already melting all over its surface during the summer and the evidence of the existence of large rivers that drain and cross all the ice mass to the rocky substrate, however, have put scientists on notice and the estimates that were made until recently of the rise in sea level from here to the end of the century have suddenly become very short. At the time it was said that the sea would rise by about 30 cm by the end of the century, but today it is not uncommon to hear scientists say that the sea level could rise by as much as 2 metres, which would already have a considerable effect in many areas. In the longer term, if all the ice covering Greenland were to melt, the sea would rise by about 7 metres. And if Antarctica were to start melting, we would then be talking about a sea level rise of up to 35 metres, which is the level it reached in the Middle Pliocene....
This map shows which areas of the SW of our country would be flooded if the sea rose 7 meters / www.floodmap.net
The region of Spain that would suffer most from the effects of the rise of the sea is undoubtedly the Lower Guadalquivir. With a rise of 7 metres in sea level, it would reach the gates of Seville and would form between the Andalusian capital and the coast a great gulf that, in reality, already existed in antiquity but was filled by the terrigenous inputs from the Guadalquivir river. If you look more closely at this map in the floodmap application, you will notice that all the towns of some importance in that area are located on the shores of that great gulf. All of them were coastal populations when they were founded and, if the melting of Greenland and Antarctica does not stop, they will become so again in a not so distant future. The National Park of Doñana would be the great damaged one, losing a good part of its extension but it will certainly be very interesting to see how new and novel ecosystems are formed in this area. Who knows if in the future, with a frankly subtropical climate, an immense mangrove swamp will not develop here. I can already imagine the Romería del Rocío progressing in boats among the roots of the mangroves...
With the rise in sea level and temperatures, the appearance of the lower Guadalquivir could change a lot in the future and look more like this...
Altitude rise of vegetation
Another logical consequence of the increase in the average temperature of the planet is the migration of species towards the poles and the rise in altitude of the vegetation altitudinal zones. To get a small idea of the extent of these changes, the first thing to remember is a basic meteorological fact: the adiabatic gradient of the atmosphere. This gradient is the difference in altitudinal temperature observed in the lower parts of the atmosphere in which we live. In a dry atmosphere, that gradient is approximately 1 degree per 100 meters. In a humid atmosphere, that gradient is approximately 0.57 degrees per 100 meters. Knowing what the increase in temperature has been in the last decades and what the forecasts are for the end of the century, we can have an approximate idea of how much the vegetation altitudinal zones will rise (at least potentially). The first question to be asked is how much the temperatures have already risen. We have already examined this in detail in a previous article (E pur si riscalda) and we then saw that the average temperature in Madrid had already risen by approximately 3 degrees. Taking the two values of the adiabatic gradient, this thus means a potential rise of the vegetation altitudinal zones of between 300 and 546 meters. This does not mean, however, that the ecosystems have already moved up to their corresponding altitudes as the readjustment of vegetation to new climatic conditions is much slower than the evolution of the climate.
Evolution of the average annual temperature in the Retiro station in Madrid (red curve) and the Navacerrada pass (blue curve) according to public data from the AEMET.
The forecasts for the end of the century are a rise of at least 6 degrees in much of the Iberian Peninsula. In other words, that would double the figures I gave you before and the rise of the vegetation floors would then be from 600 to 1092 meters. That is enormous. I have tried to do the exercise of imagining what the Central System would be like at the end of the century and this is what I can imagine:
What I've done is simply to rise the vegetation altitudinal zones. In doing so, however, questions arise about what might happen at some points such as those I have marked with a question mark. What will happen at the foot of the mountain range (2), if the vegetation altitudinal zones rise between 600 and 1000 meters ? Personally I imagine that we would have a thermomediterranean type of vegetation with lentisks, carob trees and mediterranean dwarf palms, which I have represented with a somewhat darker tone. And what will happen at higher altitudes (1) if, as expected, the level of rainfall is maintained? To imagine that the holm oaks will simply rise and occupy these levels does not seem the most likely. Perhaps species such as the cork oak, or more demanding deciduous species such as temperate oaks and maples, will take advantage of this. It is a mystery. However, if questions like these already arise in a place where evolution seems quite predictable, imagine the difficulty of predicting what is going to happen in a region such as the Atlantic seaboard. In many places the climate could become humid subtropical. And do you know what kind of vegetation grows in such places?
Laurisilva on the island of La Palma (Canary Islands) / Fotografía: Gruban / Licencia: CC BY-SA 2.0
Development of novel ecosystems
The rise in temperature and the modification of the rainfall regime could lead in some regions to the emergence of totally new ecosystems, although very similar, morphologically, to those that existed in the past. Will, for example, the temperatures rise enough for the mangroves to return to our coasts? Our coasts, in any case, have an increasingly tropical aspect, thanks to the arrival of an endless number of plants of tropical and subtropical origin whose propagation and development will go more and more throughout the centuries to come. One of the families of plants that contribute greatly to modifying the appearance of our coastal landscapes are the palm trees, which are becoming naturalized in many parts of the Mediterranean coast. The same could also be said of cactus, to which I will one day dedicate a specific article.
The naturalization of different species of palm trees in the Iberian Peninsula gives our landscapes a subtropical air that is a faithful reflection of the changes that are occurring.
As I mentioned before when talking about the rise of the vegetation altitudinal zones, the rise of the winter temperatures will favor in the humid regions the growth of many perennial species that will modify completely the physiognomy of the forests that they colonize. It may seem a distant phenomenon and imagine the "return" to the Peninsula of the species that coexist in the Macaronesian laurel forests may seem like science fiction. It should not be forgotten, however, that this phenomenon is already observed in the southern Alps, where several perennial tree species have escaped from the gardens. This is also true in Portugal, where a species like Persea indica is considered an invasive species in the Serra de Sintra. At the opposite extreme, the lack of rainfall could instead lead to the development of savannas and deserts in vast territories in the centre and south-east of the Peninsula.
The great biomes in the western Mediterranean in the middle of the Pliocene. The great differences that are observed today already existed, being even more marked the difference between the Atlantic façade and the north of ka Peninsula, where they dominated the perennial forests (laurel forests) and the center and SE, where the vegetation was similar or even more open than the current (desert in the SE). / Fauquette S. et al. (1999) / Climate and biomes in the West Mediterranean area during the Pliocene / Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 152, pp. 15–36..
The fact that we are returning to Pliocene climate does not necessarily mean that the ecosystems of the future will be absolutely similar to those of the past. The study of the past allows us to predict what the broad features of future ecosystems might be, but it is evident that these ecosystems could be very different from what they were. Many species native to distant lands will take advantage of the opportunity to expand and develop, and many of these ecosystems will be absolutely novel. New species could even appear as a result of the process of differentiation and speciazion that affects exotic species already installed or thanks to the game of hybridization. There are already several cases of absolutely new species in Europe, which raise such delicate questions as this one: can an absolutely new species born here and adapted to the current ecological conditions of the place where it has appeared be considered exotic?
The small groves of Siberian elm and trees-of-heaven that surround our cities are a current example of a novel ecosystem developed in conditions that did not occur naturally on the Peninsula.
The extent of the changes that can be anticipated already forces us to rethink many aspects of our environmental policy. Does it make sense to "pursue" exotic species in a world subjected to such important and drastic changes? Does it make any sense to want to force endangered species to stay in places that tomorrow will be unfavorable to them and where they are doomed to disappear? Until we've become aware of it, I fear that any effort, however admirable it may be, may not bear the fruit that is expected in the long term.
