Hello. In this sequence, you will understand why the climate is warming, how we know that man is responsible for it. You will learn what a greenhouse gas is. You will also understand the methods that climate scientists used to perform climate projections. Finally, you will learn the magnitude of current greenhouse gas emissions and inflections necessary to limit global warming with a particular focus on the contribution of the energy sector. The climate is warming up. The animation that follows shows the evolution of the average annual temperature of all countries of the globe when it's blue, it's cold when it's red, it's hot, cooler years, flew warmer years and vice versa. This is called natural climate variability, but we see that the blue fades away to make room for more marked red for 30 years. Global warming has accelerated today. The climate is worn by one degree C compared to the pre industrial levels, and no doubt persists. The man is responsible for this warming whose catastrophe impacts on society are already perceptible. If the climate warms up, it is due to human emissions of greenhouse gas, a greenhouse gas absorbs and emits radiant energy in the field of thermal infrared. The main greenhouse gas in the earth's atmosphere is water vapor, whose origin is essentially natural. Then there are the anthropogenic greenhouse gas. They are carbon dioxide produced by fossil-fuel combustion with the lifespan of around 100 years, Methane of agricultural origin with a life span of about 10 years and warming potential 20 times higher than carbon dioxide, Nitrous oxide byproduct of fossil fuel combustion with a lifespan of around 100 years and warming potential 300 times higher than carbon dioxide. In the absence of natural greenhouse gas, such as water vapor or ozone, the average atmospheric temperature are surface would be -18°C,, and life on Earth would not have been possible. Thanks to the presence of these greenhouse gas, the average surface temperature is plus 15° and the earth is habitable with a greenhouse gas concentration that is steadily increasing due to human activities. The lower atmosphere extending from the surface to about 10 to 15 km warms up. As the upper atmosphere cools down, the climate system is in a state of forced imbalance. The equilibrium temperature is not reached and only the stabilization of the concentration of greenhouse gas can allow to reach a new state of equilibrium. The stabilization depends on our present and future actions. How are we sure that humans are the cause of global warming? We use climate models. The figure shows the evolution of anomalies of surface temperatures compared to pre industrial levels averaged over the globe until the year 2000, three curves are visible, the black curve shows the observation. Then two other curves are derived from climate simulations. The curve corresponds to simulations in which anthropogenic sources are absent. The red curve corresponds to the simulations fed with the observed greenhouse gas concentrations. The curves correspond to the average of about 40 simulations performed in different research centers worldwide, and the envelope indicates intermodal dispersion. It can be seen that only simulations integrating anthropogenic emissions of greenhouse gas are able to reproduce observation, thus demonstrating the role of human activities in global warming. It is indeed observed that during this period the CO2 concentration increases from about 300 to about 370 parts per million. This concentration is measured at the Mona lower Reference station in the middle of the equatorial pacific, away from any local sources. In 2018, it is about four or five parts per million In the last 400,000 years and before the industrial era, the CO2 concentration oscillated between 180 and 300 parts per million, following the Milinkevich cycle of about 100,000 years. But how will this evolve in the future? The way this is going to evolve is not strictly forcible, since the trajectory will largely depend on the actions that the nations of the planet will take to reduce greenhouse gas emissions. This time we feed climate models with greenhouse gas concentrations that are no longer observed, but that come from scenarios. What defines our future is therefore largely dictated by emission scenarios that we call anthropogenic forces. But how is an emission scenario builds? The scenarios are produced using demographic and socio economic assumptions that are converted into greenhouse gas emissions. In concrete terms, a scenario is a representation of a possible future for a prospective purpose. It is exploratory, must be plausible, can be simplistic or idealist or even deliberately utopian or unacceptable. It is customary not to consider one but several scenarios. The figure shows the greenhouse gas emissions for the four scenarios adopted for the fifth IPCC report. The scenarios also include emissions or concentrations of pollutants, land use inventories and socio economic data needed to calculate the impact of climate change. These scenarios are named representative concentration pathways referred as RCP. These scenarios have been translated in terms of radiative forcing, which quantifies the modification of the radiative balance of the planet, which is expressed in what per square meters. The higher this value, the more the earth atmosphere system gains energy and warms up. The four scenarios range from 2.6 what per square meters to 8.5 what per m2. This curve shows the change in global average temperature at the earth's surface compared to the end of the 20th century. For three of the four scenarios presented previously. The curves diverge from the beginning of the 21st century in red, the most pessimistic scenario that, unfortunately, is the one that best reflects the current situation In blue. The scenario to limit global warming to 2°C in yellow. The most optimistic intermediate scenario. Depending on the scenario, the warming could be one to 4°C on average. Yeah, the two degrees Celsius limit of acceptable global warming was set in Copenhagen at cop 15 in 2009 and was at the center of the negotiations that led to the paris agreement. This threshold was proposed with reference to period of the past when this average temperature has been reached without a climatic disaster. Nevertheless, as climate change is not linear, there is no certainty that beyond an overall warming of two degrees Celsius, there will be a climate change runaway in response to phenomena still poorly appreciated. As the increase in methane emissions into the atmosphere caused by the melting of permafrost. Kocian therefore encourages us to respect the 2° to limit which is not absolute, but which, if exceeded, would increase the risk of beautification with potentially dramatic consequences. Article two of the Paris agreement expresses the commitment of the parties to maintain the increase in global average temperature well below two degrees C above pre industrial levels, and to pursue efforts to limit increase in temperature to 1.5 degrees C. Knowing that this would significantly reduce the risk an effect of climate change. Indeed, these thresholds have been discussed in terms of risks that have been quantified by the climate scientific community. The threshold of 1.5°C corresponds to a reduction of 45% of CO2 emissions by 2030 and threshold of 2°C,, a decrease of 20%. For example, in terms of water stress, an additional half billion people would be at risk of water shortages. Almost three times more people would be affected by floods, while the fish stock would decrease considerably in order to achieve the long term temperature goal set out in article to the parties driving article for to achieve a global peak in greenhouse gas emissions as quickly as possible. And this, in order to achieve a balance between sources and sinks of anthropogenic emissions during the second half of the century. Until 1970, about 1000 gigatons of CO2 were limited, it doubled over the next four years. Two generations of humans have double the CO2 concentration today, the emission rate is around 40 gigatons of CO2 per year. Now consider the carbon budget for the 1.5°C.. Global warming limit until 2011 1900 gigatons of CO2 were released. The right to limit up to the limit of 1.5° C. Is 550 gigatons at the current rate it would be reached in 14 years at the cop 21 all nations were required to submit national contributions or intended nationally determined contributions, abbreviated as I M D C I M D C S. Our national targets for reducing greenhouse gas emissions present the budget would be exceeded by 240 gigatons of C02 in 2013. Now consider the carbon budget for the two degrees Celsius. Global warming limit. The right to emit up to the two degrees C limit is 1000 and 10 gigatons. Given the submitted by NBC's, the remaining budget in 2030 would be 210 gigatons of co. II at the current rate it will be reached five years later. In summary. The iron disease would be emitted in 25 years, which is tomorrow at the time scale of public policies and energy strategies. The energy sector accounts for 35% of greenhouse-gas emissions, reducing our greenhouse gas emissions to mitigate climate change therefore necessarily means a profound transformation of the system of production and distribution of energy production and a revolution in our behavior in energy consumption. In this sequence, we saw the main mechanisms behind global warming and explain how the man is responsible for it. We have explained the origin of the thresholds of 1.5 degrees and two degrees C of global warming. At the heart of climate change negotiations on climate change, maintaining the two degrees C trajectory of global warming requires reducing our greenhouse gas emissions by 50% in 2050 compared to 2010. Given that energy accounts for 35% of emissions, It also requires carbon neutrality. Between 2050 and 2100. Thank you for your attention. [MUSIC]
