In most industrialized nations, the debate regarding the causes of climate change caused by human activities revolves around emissions of greenhouse gases such as CO2 or methane.
These gases prevent part of the energy from the sun that passes through the atmosphere from returning back to outer space, causing a greenhouse effect that is increasing the global average temperature, causing an increase in the frequency and intensity of extreme weather events such as floods. and droughts, and making the climate, ultimately, more unpredictable and changeable, and less suitable for the development of activities such as agriculture, on which our survival depends.
However, on many occasions we forget that deforestation also plays a very important role as a determinant of climate change. The cutting and burning of the forests that cover a large part of the planet's surface releases CO2 into the atmosphere, helping to increase the concentration of this gas in the atmosphere, already important due to the burning of fossil fuels. On the other hand, the disappearance of forests has important consequences in the biophysical processes related to the water cycle. Tree leaves release water into the atmosphere through a process known as evapotranspiration. This water vapor helps to cool the environment, and in places where the forest cover is extensive and the trees are large, it can even generate precipitation. It is what meteorologists call "convective rains." All this contributes to a relative cooling in areas where the area covered by forests is greater compared to those located in similar latitudes where it is less.
Historic deforestation has contributed to promoting climate change in mid-latitudes. In the photograph, cultivated fields and hills in the vicinity of Peñafiel (Valladolid).
Despite the importance of deforestation in explaining changes in the current climate, the effect of changes in land use that have taken place in the past on climate change has been generating a widedebate within the scientific community. During these years ago, some studies have suggested that the effect of historical deforestation has generated a cooling of the annual average temperature that has contributed to mitigate global warming, especially in mid-latitudes. The results of these studies are in line with those of the fifth (and last) IPPC report, which show a negative radiative forcing, that is, a tendency to decrease the global mean temperature due to the increase in albedo (the percentage of radiation incident solar that is reflected by the earth's surface) that has taken place in mid-latitudes due to deforestation. However, in recent years, several experimental and model-based studies show that the main effect of deforestation is an increase in seasonal and diurnal thermal oscillations.
How have changes in land use affected the changes that have occurred in the climate of the different regions of the earth? Has historical deforestation mitigated or promoted climate change? The team led by Quentin Lejeune, a scientist at theInstitute of Climatic and Atmospheric Sciences of the ETH of Switzerland, in a study published last April by the prestigious journalNature Climate Science. In this study, the authors have reconstructed the historical highs of temperature around the world during the period between 1861 and 2000 through simulations with the available climate models. These simulations have considered the effect of deforestation on maximum temperatures through the release of CO2 and its impact on processes related to the water cycle. When evaluating their results, the authors have also taken into account the effect of other factors that could influence global temperatures, such as greenhouse gas emissions, the concentration of aerosols or suspended particles, and eruptions. volcanic.
The results of the work show that deforestation throughout the study period has caused an increase in maximum temperatures in middle latitudes. In theNorthwest America, theEuropean Russia and some countries ofEastern Europe, where the deforestation rate has been greater than 15%, the increase in maximum temperatures and the number of warm days throughout the year has been particularly important. In these regions the reduction of vegetation cover is responsible for up to a third of the temperature increase since the pre-industrial era. Another interesting finding of this work is that, during the first decades of the study period, at the beginning of the industrial revolution, the increase in global average temperature was explained by deforestation and not so much by the (still incipient) increase in concentration. of CO2 and other greenhouse gases in the atmosphere.
Another very interesting result of the work is that some tropical areas that have been heavily deforested during the second half of the 21st century, such as theAmazon basin, have seen their maximum temperatures decrease. This is because in these cases, the forest cover, which has a very low albedo, has been replaced by crops and pastures, which have a very high albedo, which has caused a slight cooling. In North America and Russia, however, much of the deforested areas have been replaced by urban and industrial areas, whose albedo is quite low, so the general effect has been to increase maximum temperatures.
The results of this work show us that changes in land use have no less effect on climate change, and therefore should be considered when studying past and future changes in climate, particularly in relation to extreme temperatures. Likewise, they also underline the potential of increasing forest area such as reforestation (recovery of plant cover in areas previously covered by forests) and afforestation (planting trees on degraded lands) to reduce the increase in maximum temperatures with this, mitigate the effects of climate change.