For the first time, the human origin of global warming is directly demonstrated

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Work carried out by researchers from the NASA directly demonstrate the link between our greenhouse gas emissions and global warming. Thus, for the first time, it was possible to prove by observations the anthropogenic origin of the current climate change. The results are published in the journal Geophysical research letters this March 25.

The increase in atmospheric concentrations of greenhouse gases (GHG) linked to human activities induces an accumulation of heat on Earth. Indeed, these compounds prevent the planet from cooling easily. Going through the emission of infrared radiation into space. In short, greenhouse gases act like an insulating layer the efficiency of which depends on the quantity of gas in question.

Global temperature anomaly between 1880 and 2018 according to different datasets. Credits: NASA GISS / Gavin Schmidt.

By increasing it, humanity thus causes an imbalance between solar energy which enters the climate system and that – infrared – which leaves it. Therefore, an overall temperature change occurs. A dynamic destined to continue as long as a new equilibrium is not reached. However, although physicists have understood the link between human emissions of GHG, the resulting disturbance of the Earth’s radiation balance and climate change, no direct observation of this effect existed until now.

Heating and GHG : a directly observable relationship

And for good reason, for several decades, it was mainly physical calculations – coupled with laboratory experiments – that made it possible to be convinced of this. Over the past twenty years, the increasingly significant manifestations of a warming similar to that anticipated have added additional coherence. But without being able to speak of a direct observation of the link between the increase in GHG and global climate change. A gap that has now been filled thanks to the work of a group of researchers from the NASA.

To do this, the scientists used in particular the satellite measurements accumulated over several decades within the framework of the project. CERES. These data characterize the energy flows entering and leaving the climate system. However, if the measurements do show the presence of an imbalance consistent with warming, a simple reading does not allow it to be attributed to a particular cause. To counter this, the researchers used a technique called radiative nucleus trick. This is a complex method of analysis that we will not explain here, but which made it possible toisolate the anthropogenic contribution from natural influences in the observed energy imbalance.

climate warming
Emitted infrared radiation (left) and reflected solar radiation (right). An illustration of the many products resulting from satellite measurements. Credits: NASA.

Precise quantification based on satellite observations

At the end of their effort, it appeared that human activities caused a nominal imbalance of 0.5 W / m² between 2003 and 2018. In other words, almost all of the radiative disturbance driving global warming. The study shows that the anthropogenic increase in GHG is the dominant contribution, with also a not insignificant role of the decrease of the particles of pollution in the air. Indeed, this dust tended to reflect solar radiation and to cool slightly. It should be emphasized, however, that these results are not fundamental in our understanding of the influence of GHG. A bit like the first photograph of the Earth was not to be aware of its sphericity.

“This is the first calculation of total radiative forcing on Earth using global observations, taking into account the effects of aerosols and greenhouse gases”, note Ryan Kramer, lead author of the article. ” It is direct evidence that human activities are causing a change in the Earth’s energy balance. In addition, the method has the advantage of allowing rapid monitoring of the evolution of the energy imbalance. An advantage compared to the previous ones which required cumbersome procedures due to the use of digital modeling. “The creation of a direct record of radiative forcing calculated from observations will allow us to assess to what extent climate models can simulate these forcings”, adds Gavin Schmidt, director of GISS to the NASA.

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