Welcome to this lesson on working with scenarios. Focused theories and models, projections are scenarios which are used to explore possible future climate change. Climate change is one of the overarching global issues and in many ways, perhaps one of the most or the most pacing one. Even though climate change is only one, number 13 of the sustainable development goals, it is multiply connected one way or another to pretty much all of the other goals. For example, such land-based alternatives, the mitigation to reduce fossil emissions, as bio-energy with carbon capture and storage has implications to goals which are about food security and ecosystems. In turn, present of the goals which are about sustainable energy system, production and consumption, and so forth, are prerequisite for a future for [inaudible] and climate neutral future. In this context, models summarize our understanding of the system under study, and they enable exploration or the effect of alternatives to deal with climate forcing, climate change, and its impacts. For example, to help us to make good decisions and climate goals. The latter concern both adaptation to climate change that it's not devoided by mitigation, and mitigation which is limiting future greenhouse gas emissions and unsustainable land-use change. In this way, [inaudible] provides us with the means to stay ahead of things as well as to chose what kind of future we want to aim at. In climate change science, there's a range of models which address different aspects. Integrated assessment models address driving factors behind greenhouse gas emissions, such as changing energy technologies, energy use, land use change, and relevant to sell their choices and trends, all based on empirical data and economic theory. Climate models, on the other hand describe the physical climate system, its characteristics and processes, and as such, also how the system changes and as enforcing that affects the energy balance of the earth, thus triggering a change in climate, the solar variability or anthropogenic greenhouse gas emissions. Climate models have been around from at least the 1960s and they have been steadily developed since then. Today, they provide a good description of the climate system when compared to observations, and also reasonable response of climate system change when run for the climate periods in the past that we have information on. They are both global climate models and regional climate models, the difference being the geographic domain addressed. They are also different simplified models used for climate system studies and the comprehensive climate, or nowadays how they are called earth system models. The letter are based on fundamental physical laws, as on denti-fluid mechanics, thermodynamics, and ready to transfer, which collectively give rise to a set of equations that can be run a powerful computers together with a set of boundary conditions such as greenhouse gas emission scenarios. The physical basis of climate models and the evidence and how they capture climate systems behavior under forcing, mixing the primary scientific method for exploring possible future climates under different alternatives or anthropogenic forcing. This involves projecting climate change with a climate model under some anthropogenic scenario of greenhouse gas emissions and land use change, which are the primary anthropogenic climate forcing factors. The resulting time series of climate system data, such as an global mean temperature and sea level change, is sometimes also called a climate scenario. Climate models are often run in large coordinated experiments and for a wide range of different force and scenarios to explore alternative future climate states and evolutions. The models calculate the response of the climate system, including relevant feedback, for example, how clouds and their properties respond to initial warming and change it into water vapor content on the atmosphere and so on. One measure of the total effect, the different feedback, it's so-called climate sensitivity. Overall, such changes within the climate system strengthen the initial response to emissions, which is a positive feedback. Thanks to the physical betas are climate models, they also gone for interveneability within the climate system, such as what is characteristic for the dynamics and interplay of atmospheric and ocean circulation systems. Climate scenarios inform about the consequences of different emission developments or development pathways. They also provide framing for researching climate impacts, that is how climate change affects natural of human systems. For example, agriculture, ecosystem services, and risk for extreme events. Such studies in turn inform adaptation to climate change, but also study some measures to pursue other sustainable development goals where climate change is a factor. Changes the condition for agriculture for example, affect the challenges in food security. Another line of climate modeling is to explore scenarios which target a specific climate goal, such as the two-degree and 1.5 degree targets mentioned in the Paris Climate Agreement from 2015. This involves performing a number of climate model runs with a specific very low or low emission scenario, such as the global carbon dioxide emissions culminating in the next few years, to rapidly decrease the net-zero at the mid-century, followed by the so-called negative emissions, which means removing carbon dioxide from the atmosphere. Such development pathways, sometimes also called storylines, are often constructed with the integrated assessment models in which possible socioeconomic choices and possibilities are chartered in a consistent manner. By doing so with many integrated assessment models are many climate models provides estimates for the probability of reaching the target in question with a specific emissions pathway or the specific set of decisions. This informs efforts on climate change mitigation. As some mitigation alternatives are land-based, for example forestation, reforestation and afforestation, as well a bio-energy, and others rely on technological and behavioral changes collaboration across countries, development institutions such like this again, a clear connection, many clear connections to other sustainable development goals. Different mitigation alternatives meet either presents sooner cheese or goal conflicts within this overall agenda 2030 frame. Scenario so inherent in exploring the future before we actually get there. Of course, the probability of the real word following a specific scenario is not null, except indicates that the decisions are made to that end. The Paris Agreement has set global climate cause or limiting global warming to well below two degrees Celsius, preferred up to 1.5 degrees Celsius at most. Obviously, the more ambitious the goal is, the more challenging it is to bring into the vault. But also coming with the benefit of limiting the consequences of climate change to society and ecosystems. Today, it remains to be seen if and when all the corresponding decisions to sufficiently reduce the global climate emissions are taken, and whether it will be done fast enough. So far the base has been insufficient, but they are science into world of an increasing action and ambition. The Paris Agreement goals do not prevent all climate change impacts if the goals are met, but they do limit the consequences compared to larger degrees of warming. Still, even if the goals will be met, there may be surprises in the weight if the climate system or the humans system respond in unexpected ways of change. In the climate system, this is so-called tipping points. That is onset of rapid and possibly irreversible changes in some parts of the climate system after a certain level of forcing or chains has been reached. Typically, and even though tipping points have occurred in the past, it is not known with certainty how much global warming is too much for some part of the system that can reach a tipping point. The consequences however, can be studied in climate models in designated experiments. Scenarios are a powerful tool in charting possible developments and testing different decision and development alternatives against societal goals, such as in the case of climate change. Even though we actually are in the middle of a global warming experiment with a real system itself, the future is still amenable, the choices, how we do things. Nevertheless, it should always be kept in mind that to understand a scenario requires understanding the assumptions behind the scenario and the tools that had been used to translate the assumptions into an outcome be it by means of a climate model or something else.
