The Sixth Assessment Report of the United Nations IPCC (Intergovernmental Panel of Experts on Climate Change) has just been publicly presented. The document includes the advances in scientific knowledge on global climate change compared to what was available in 2013, when the previous report was published.
The evaluation presents new evidence that, even more overwhelmingly than in the previous ones, blames various human activities for this serious problem. Among them, the progressive accumulation in the atmosphere of certain greenhouse gases emitted stands out, which causes an evident global warming of the earth’s surface that alters other factors of the climate system.
The most relevant aspects of the current climate assessment are:
- that global warming of the earth’s surface is accelerating (0.2 ℃ in the last decade versus an average of 0.12 ℃ per decade between 1950-2011),
- that a similar behavior is observed in other climatic factors (mean global increase in rainfall, reduction in continental ice and snow cover and rise in mean sea level),
- and, the greatest novelty, that extreme climatic events (heat waves, torrential rains, periods of drought) have increased so much in frequency and intensity that in various regions of the planet they no longer allow a possible explanation without human influence.
All of the above is detailed by rigorous analysis of a massive set of unambiguous observations on a planetary scale.
What will happen on the next years?
These alerts that scientists launch about what is happening are sharpened by showing the results of a set of climate change projections until the end of the century. They have been made with dozens of even better and more sophisticated physical models that simulate the behavior of the various components of the climate system.
To do this, they have considered five hypothetical scenarios on the possible future evolution of the global volume of human emissions of greenhouse gases responsible for altering the climate.
- The first assumes a rapid increase in emissions, doubling by the middle of the century,
- in the second that they doubled at the end of the century,
- in the third, they remain similar until the middle of the century and then decrease moderately,
- in the quarter that decrease from the present until reaching a net value of zero in the last quarter of a century
- and in the fifth that this net annulment is reached in the middle of the century.
It is obvious that none of these scenarios will coincide with what actually happens throughout the century. However, it is the only procedure available to estimate the socio-economic risks associated with future climate change that humanity would have to face depending on how global emissions evolve.
Current climate models successfully reproduce the main changes experienced by the global climate in the last two centuries. Of the numerous results of the simulations, the following should be highlighted:
It is confirmed that the main contributor to future warming is CO₂ (more than 75%). It is followed at a distance by methane (CH₄) and nitrous oxide (N₂O). Therefore, mitigating measures must prioritize the reduction of CO₂ emissions, which almost exclusively come from the use of fossil fuels.
Of the five emission scenarios considered, only the most optimistic would be able to limit global warming to the value that the Paris Climate Agreement indicates as desirable (1.5 ℃ with respect to the pre-industrial climate). Therefore, it could be said that it is highly unlikely to achieve this. Only the countries of the European Union and a few more contemplate reaching a net zero value of CO₂ emissions by mid-century.
And only in the following scenario (reaching zero net emissions from 2075) would it be possible to limit global warming to 2 ℃, the value promised in the Paris Agreement.
In the other three cases, the end of the century would be reached with higher global temperature increases, which could reach increases of over 4 ℃. Temperatures that the planet has not experienced in at least the last 3 million years, according to all available paleoclimatic evidence.
Future increases in the frequency and intensity of extreme climatic events, which cause the most disastrous effects, are directly related to the increase in global average temperature. In other words, even in the most optimistic emissions scenario, the extremes that are now being observed would worsen, and without a doubt much more so in the other scenarios. The report shows with illustrative examples the expected changes in frequency and intensity of heat waves, torrential rains and periods of drought in many regions of the planet.
Can we really reverse climate change?
One aspect that tends to stand out a lot is the degree of “irreversibility” presented by current or future climate changes. First, it would be necessary to clarify when it is considered that it “reverses” a process of change. Only when it returns to the initial conditions or after the sign of its trend changes.
The observed global warming is conditioned by the accumulated volume of greenhouse gases emitted since the pre-industrial era. Therefore, it would only begin to decline (change trend) after global emissions are netted out. In other words, it could only be achieved in the two most favorable hypothetical emission scenarios.
However, it would take a few centuries for the global average surface temperature to return to pre-industrial values after greenhouse gas emissions completely ceased. The main cause is the effect of the components of the climate system with greater thermal inertia (continental ice and the deep ocean). Therefore, they would be irreversible for many generations.
In summary, with this sixth report, once again the scientific community warns with very solid arguments to the rulers, the economic powers and the entire society of the serious risks that would inevitably derive from a greater delay in the adoption of measures to reduce drastically the emissions generated by the use of fossil fuels. This is not catastrophism, but knowledge and responsibility.
Manuel de Castro Muñoz de Lucas, Professor of Earth Physics, University of Castilla-La Mancha
This article was originally published on The Conversation. Read the original.
