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Climate change. What is the true weight of the human factor?

Is the human being responsible for the current climate change? Absolutely yes. Moreover, being honest, we would have to say that this question should not be in this article. At least, it should not appear if we talk about current debates, since today there is no doubt about it.

In the latest report from the Intergovernmental Panel on Climate Change (IPCC) we can read that “it is unequivocal that humans have warmed the climate system since the pre-industrial era” . If we look back at these reports we will see that such attribution has always existed and, over the years, what has happened is that the certainty that this was so has increased. Proof of this is the second IPCC report, published in 1995, which indicated that there were limitations to clearly detect the anthropogenic signal in the climate, but that “the body of evidence suggests that there is a discernible human influence on global climate ».

Today it is undeniable that this is the case and, although it may surprise us, the foundations that lead us to understand what is happening and what can happen go back a long, long time.

The IPCC is the UN body in charge of evaluating scientific, technical and socioeconomic publications related to climate change. Thousands are taken into account to prepare their reports and hundreds of scientists who participate in this work. Moreover, the final result is born from a consensus in practically every published word. In this way, the IPCC reports show us a very clear picture of the established science in this field to date. They are not just any source.

A century until the IPCC

Back in the 1850s, the first studies on the role of CO2 in heating the atmosphere were carried out. But it would be necessary to wait until 1896 to find the first publication in which it appears how the planet’s temperatures would change according to different concentrations of this gas. That article is On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground and its author is Svante Arrhenius (and yes, “carbonic acid” because at that time CO2 was called that).

Among other things, Arrhenius showed in this work how global temperatures would change as a function of the concentration of CO2 in the atmosphere . This last quantity was expressed in variations with respect to the concentrations at that time and he obtained that, with half the CO 2 in the atmosphere, the temperature would drop between 4 and 5 ºC. If the concentrations were increased by 50%, the temperature would rise by 3-4ºC, and if they doubled, the increase would be between 5-6ºC (note: it should be noted that their results were very accurate considering the rudimentary data that was reported). used and, above all, that were calculated more than a century ago).

This article also mentioned that the burning of coal was linked to the emission of CO2 into the atmosphere but, although it is currently a widespread idea, it did not warn about the warming that this effect could produce. First, the motivation for this study was not related to global warming, but to understand ice ages! Furthermore, the text alluded to the conclusions of a geologist of that time, Professor Arvid Gustaf Högbom, in which it was indicated that the amount of CO 2 emitted into the atmosphere by coal was compensable by physical processes that they form carbonate (however, at the beginning of the 20th century it was already indicated that the burning of coal would generate heating).

It’s been a long time since then. Our knowledge of the climate system has increased considerably and, simultaneously, the amount of CO2 in the atmosphere. At the end of the 19th century, the concentrations of this gas were around 295 ppm, a value that no living human being has experienced now. Not even the oldest person in the world, who had just turned 119 at the time of writing this article, has done so. Since the birth of this woman in 1903 (the same year that Arrhenius was awarded the Nobel Prize in Chemistry, by the way), CO2 concentrations in the atmosphere have varied from about 297 to almost 420 ppm. An increase of 40% that has occurred at a rate that, surely, the scientists of that time would not have believed.

carbon cycle

CO2 is emitted and absorbed in nature by different sources . If we take a look at the annual flows of this gas in nature, we will see that human beings emit around 5% of what the oceans and vegetation emit together. In other words, our emissions are much lower than those due to other natural sources. This could lead us to think that the impact of human beings on the increase in CO2 in the atmosphere is negligible, but then we would be left with a half-hearted argument: vegetation and the oceans are major emitters of CO2, but they are also major sinks. In fact, they absorb more CO2 than they emit.

Even so, the hypothesis that was considered in the publication of Arrhenius more than a century ago could be true. That is to say, it would be possible that this net absorption by the vegetation and the oceans was capable of offsetting the emissions of the human being. However, we now know that natural processes are not being able to offset the CO2 emissions from fossil fuels . They are not doing it now and they were not doing it then. If this had been the case, CO2 would not have been accumulating in the atmosphere since the mid-nineteenth century to reach unprecedented amounts in the history of human beings as a species.

In addition, the fact of having added more CO2 to the atmosphere has caused some natural emission and absorption fluxes to change. For example, the ocean is absorbing more CO2 now than it was a few decades ago. Thanks to this effect, the concentrations of this gas in the atmosphere are not increasing at a faster rate, but it is causing the oceans to acidify (another consequence of climate change), a great risk for marine ecosystems.

In any case, looking again at the proportion of CO2 that human beings emit with respect to other sources, can we ensure that this CO2 comes from our activities? The answer is yes and through such curious aspects as, for example, the carbon isotopic footprint. What makes an atom of one element and not another is the number of protons it has in its nucleus. In this way, hydrogen has only one proton, lithium 3… and carbon 6. However, in the nucleus we can find a different number of neutrons and this means that the atoms of the same element can have different masses, even though they occupy the same place in the periodic table.

That is what we know as isotopes and, in the case of carbon, there are three that we can find in nature: carbon 12 (with 6 protons and 6 neutrons in its nucleus), carbon 13 (with 6 protons and 7 neutrons) and carbon 14 (with 6 protons and 8 neutrons). From that list, carbon 14 is surely familiar to us because of its use to date samples, a procedure that takes advantage of the fact that it is transformed into nitrogen 14 over the years. It is a radioactive isotope, but carbon 12 and 13 are not, so they do not disappear over the years. It turns out that in the atmosphere there should be certain and relatively stable concentrations between carbon 12 (majority) and carbon 13 (very minor). This proportion between carbon 12 and 13 is similar both in the atmosphere and in the oceans, but not in the vegetation, which have an even lower proportion of carbon 13, since they basically absorb carbon 12 when doing photosynthesis, which is less heavy.

Now, fossil fuels are the result of the decomposition of plant beings, so it is expected that their proportion of carbon 12 versus 13 is greater than that in the atmosphere. Therefore, if we manage to release that carbon into the air, we would expect the proportion of carbon 13 to decrease (as if we were diluting it into more carbon 12) and this is precisely what is being observed around the world.

This is only one of the evidence that exists to link the increase in CO2 in the atmosphere with human activities. But there are many more that point in this direction: direct observations, analysis of ice cores, tree rings… and the most powerful stems from our technological development: climate models.

The models

If we can currently quantify the degree of responsibility that human beings have in climate change, it is, without a doubt, thanks to climate models. These tools have improved a lot in recent decades and are currently capable of reproducing what we are observing quite faithfully .

What’s more, we can play with them to see, for example, what would happen if we remove CO2 from the atmosphere, or if we double it… or if there were no human influence. Precisely, this is one of the proofs that without the action of our species, unprecedented temperatures would not be occurring in at least the last two millennia.

If we try to reproduce with models the evolution of temperatures since 1850 only by means of natural factors, the result does not coincide with the observations. It is necessary to include forcing due to human activities to achieve this.

This is very powerful proof of the great weight that human beings have had and still have in current climate change , but can we put it in numbers? Again, the answer is yes. In the summary for political authorities of the sixth IPCC report, published in August 2021, it is shown that between 2010 and 2019 the global temperature had risen between 0.9 and 1.2 ºC compared to the pre-industrial era (there is always a margin of error). Well, of that value, the human being would have been responsible for an increase between 0.8 and 1.3 ºC, while the natural forcings, from -0.1 to 0.1 ºC. In other words, human beings are responsible for practically all of the observed warming, while natural cycles would have contributed, at most, one tenth.

But this impact on temperatures does not arise solely from our emission of greenhouse gases (if it were only for them, the planet’s temperature would have already risen by around 1.5 ºC), but from the combination of these with other factors. For example, through changes in land use or the emission of aerosols, particles that are capable of reflecting the sun’s rays and tend to cool the Earth, as occurs, for example, with the eruption of some volcanoes.

Can we contain it… if we stop emitting CO2 right now?

At this point we are probably wondering what would happen if we stopped emitting CO2 right now. Could we solve climate change that way? This is where one of the bitterest surprises of the climate crisis appears: if we stopped emitting CO 2 right now, nothing would happen , at least not appreciably in the near future, and in some respects neither in the middle… nor in the future. far.

When CO2 is emitted into the atmosphere, some of it is quickly distributed between the atmosphere, the upper layer of the ocean and vegetation. But another percentage takes much longer to be absorbed by the soil, the rocks or the bottom of the oceans… of the order of centuries and even millennia.

To see the magnitude of these processes and the difficulty of reversing climate change, it is very useful to make use of Figure 12.44 of the 5th IPCC report. In general, by stopping emitting CO2, the concentrations of this gas would decrease slightly in the following years, but would remain without great variations for centuries.

If we put ourselves in the hypothetical case that this would happen with 450 ppm in the atmosphere, the concentrations would take practically a millennium to return to levels similar to the year 2000. The temperature would not change too much either and even some processes would continue during all that time, such as the sea level rise, as the ocean would continue to absorb heat and thus expand. But if the cessation of CO 2 emissions comes later and it continues to accumulate in the atmosphere, it will be much more difficult to return to levels simply known in the most recent history of humanity.

What if we remove CO2 from the atmosphere?

But what if we lend a hand to eliminate atmospheric CO 2 faster? There are two ways to achieve it. The first is naturally, like planting trees . This option carried out well can have environmental benefits such as the recovery of degraded ecosystems, social benefits by involving communities in the region… but there are limitations and risks such as massive planting of trees can degrade the soil, it can introduce invasive species without a proper study of the area and Furthermore, trees do not absorb CO 2 in an unlimited way.

And what about artificial techniques? Are they science fiction? No, they are not. There are artificial methods of carbon capture , but they are not sufficiently mature techniques at present. In any case, it is likely that they will be in the not too distant future and even future scenarios in which they will be operational are contemplated.

However, this possibility is not a miraculous solution to the climate change we are experiencing. If we were to absorb some of the CO2 in the atmosphere, it would take some time for some components of the climate system to find out that this had happened. For example, global temperatures would start to drop after a few years, as there is a lag between the amount of CO2 in the atmosphere and the response of temperatures.

Furthermore, such a cooling trend would not be immediately detectable. Why? It is natural variability that has the answer: just as we are not seeing a year-on-year rise in global temperatures, but some years are warmer than others, we would not see a linear decline and the trend towards cooling would be masked for a few decades.

The permafrost would also take time to find out. Even if temperatures started to drop, more of this frozen ground would still be lost and it would take a few decades for us to recover the extent before CO2 was removed from the atmosphere. Likewise, the ocean would continue to acidify for centuries and sea levels to rise for that time and even millennia.

A hard machine to stop

All this shows us that the climate system is more complicated than we may think at first. Each step is progress that is difficult to reverse, and there are certain thresholds at which some aspects of the climate system pass into a practically irreversible state. This is the reason why it was agreed in the famous Paris Agreement to limit warming to 2ºC and, if possible, 1.5ºC at the end of the century compared to the pre-industrial era.

Going back to what we just raised earlier, could we reach the limits of the Paris Agreement by absorbing CO2 from the atmosphere? Again: yes, but it is not a miracle solution. The fact of exceeding these thresholds would cause some processes to start up and take decades or centuries to return to the levels we know today. In short, the more CO2 we emit into the atmosphere and the more we have to eliminate, the more processes will start up and the longer it will take to return to currently known situations.

And we could not miss this great opportunity to mention another very controversial technique in recent times to limit the rise in temperatures: disperse particles in the atmosphere that reflect solar radiation . This would limit global warming, but it would not prevent the advancement of some consequences that we are suffering from climate change. By itself, this method would not prevent CO2 from accumulating further in the atmosphere and therefore would not stop the acidification of the oceans. But, in addition to these associated risks, it is not very well known what specific effects these substances could have on the atmosphere on a planetary scale or other consequences that are currently not known or within our reach.

Be that as it may, we may need to use artificial techniques in the future, but we should not trust the success of our goals to techniques that are not yet mature enough, that may have unknown risks on a large scale, or simply do not exist. . Much less, if we still have time to avoid more serious consequences of climate change without them.

We are on time

Scientifically it is still possible to limit the temperature rise to 1.5ºC by the end of the century. To do this, emissions need to be zero around 2030 . Let’s not fool ourselves, it is a very ambitious and difficult goal that requires a profound transformation in aspects such as the way we acquire our energy, move around, feed ourselves, consume… But they are exactly the same measures that we would have to take if we want to limit the heating at 2, 2.5, 3 ºC …

All this justifies that on many occasions current climate change is referred to as a “climate crisis” or “climate emergency”: urgent measures are needed against this problem because, the longer we take to implement them, the more serious the consequences will be.

These are the cards that science shows us at this time and the rules of the game, although winning this game does not depend only on them, but on how the rest of civilization decides to play: from politics, economy, society… one with its different number of cards and with its plays. But, beware, the less the clock advances, the less all humanity will lose and, if we want to stay within the limits of the Paris Agreement, this decade is decisive. We will make it? We will only know in the future…, for now, uncertain.

 

Isabel Moreno is a physicist. Master in Meteorology and Geophysics. Climate change expert.

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