A global climate 1.5° Celsius degree warmer: how does the IPCC consider it will affect the world?

1. How much has the planet warmed since the pre-industrial period?

Human activity is estimated to have brought about a warming of 1.0°C above pre-industrial levels. If current trends continue, a warming of 1.5°C is likely to be reached between 2030 and 2052. Some regions, like the Arctic, experience greater warming than the average, and warming of land tends to be greater than over oceans. Changes in weather and climate extremes have already been observed in the last decades, a period over which global temperatures have increased by 0.5°C.

Warming from emissions of greenhouse gasses by human activities (CO₂ but also chlorofluorocarbons (CFCs), methane, solvents, …) that have happened since the pre-industrial period will continue to cause changes in the climate for centuries to come, for instance by increasing droughts intensity and frequency, the increase of acidification¹ and level of the seas and oceans.

According to the IPCC report, if there were no more greenhouse gases emissions linked to human activities from now on, global mean temperatures would be unlikely to reach 1.5°C. This is of course not a realistic possibility since, unless decisive action is taken, current human activity will continue to cause very significant emission of greenhouse gases in the foreseeable future.

Reducing net CO₂ emissions to zero and maintaining that level would only stop warming within several decades. The maximum temperature will thus depend on the total amount of greenhouse gases that are released into the atmosphere by human activity until then. On a longer time scale, a net negative level of CO₂ emissions would be required to prevent further warming, reverse ocean acidification, and to minimize sea-level rise.

¹ Water acidification has a significant effect on aquatic ecosystems such as plant and animal plankton or coral reefs

2. What are the main risks brought by a global climate warming of 1.5°C or beyond?

Specific risks related to a global climate warming depend on the rate and amount of this global warming, but also on the location and the situation of human populations, which includes the level of development (such as the level urbanization) and vulnerability, and on the adaptation and mitigation measures that are put in place.

Climate models show clear differences, and thus risks, between the current situation of 1°C mean temperature rise and a future situation where the global mean temperature has warmed up to 1.5°C or 2°C. Besides hot extremes in inhabited regions, changes in precipitations, with some regions experiencing heavier precipitations and other experiencing more severe droughts include:

• Increase of sea levels by several meters, which could happen if warming reaches a point where the Greenland or Antarctic ice sheets are destabilized and melt. This could happen at a warming level between 1.5 and 2°C;
• Impacts on ecosystems and biodiversity, whether on land or in the oceans, are projected to be more severe at 2°C than at 1.5°C. This includes risks of extinctions, of forest fires and of transformation of ecosystems;
• Climate-related risks to health, livelihoods, food security, water supply, human security, and economic growth are projected to increase with global warming of 1.5°C and increase further with 2°C. Poverty and disadvantage are expected to increase in some populations as global warming increases.

More adaptation² will thus be needed at 2°C than at 1.5°C increase. The problem is that there are limits to adaptation strategies and, at some point, some strategies will fail, even more so for the systems and for people who are the most vulnerable.

The avoided climate change impacts on sustainable development, eradication of poverty and reducing inequalities would be greater if global warming was limited to 1.5°C rather than to 2°C. Climate change impacts and responses are indeed closely linked to sustainable development, which balances social well-being, economic prosperity and environmental protection. The United Nations Sustainable Development Goals (SDGs), adopted in 2015³, provide an established framework for assessing the links between global warming of 1.5°C or 2°C and development goals that include poverty eradication, reducing inequalities, and climate action.

² Adaptation to climate change refers to strategies, initiatives and measures to reduce the vulnerability of natural and human systems to the (present and expected) effects of climate change. The word adaptation evokes an ability to adjust, and thus a dynamic or even evolutionary vision of the functioning of societies (Wikipedia)
³ www.un.org/sustainabledevelopment/development-agenda/ 

3. By how much should CO₂ and greenhouse gases emissions be reduced to limit the global warming to 1.5°C?

For limiting temperature raise to 1.5°C limit in the total “carbon budget”, which is the total amount of CO₂ emitted by human activities since the pre-industrial era, should not go beyond 2800 Gt CO₂. As of 2017, this budget has already reached 2200 Gt CO₂ and about 42 Gt CO₂ more per year are still emitted. In order to prevent overshoot, CO₂ emissions need to decline by about 45% of 2010 levels by 2030, and reach net zero by 2050.

In short, there are two main general ways to limit the global warming to 1.5°C:

• Either the net greenhouse gases (GHG) emissions are reduced greatly so that global temperature slowly rises to 1.5°C and stabilizes there without overshoot. Pathways limiting global warming to 1.5°C with no or limited overshoot would require rapid and far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings), and industrial systems;
• Or these emissions are reduced less quickly and there is an overshoot of this limit: the global temperature then will rise above 1.5°C, and a period of net negative emissions (or emitted “carbon capture”) would be necessary afterwards to bring down the temperature rise to 1.5°C.

The lower the carbon emissions in 2030, the lower the challenge in limiting global warming to 1.5°C after 2030 with no or limited overshoot. Avoiding such overshoot and to have to rely on future large-scale deployment of carbon dioxide removal (CDR) strategies can only be achieved if global CO₂ emissions start to decline well before 2030.

4. What should be done in specific sectors for transitions towards “zero carbon” emissions?

There are 4 domains to which priority of action should be given:

1. Energy systems: Lower energy use is one of the major ways to reduce carbon emissions. Besides, fossil energy sources with lower emissions or with carbon capture solar energy, wind energy and electricity storage technologies have substantially improved over the past few years, and the political, technical and economic feasibility of these systems has also improved.
2. Industry: CO₂ emissions from industry need to be reduced by about 65–90% lower in 2050 relative to 2010, reductions which can be achieved through combinations of new and improved existing technologies and practices.
3. Infrastructure: The urban and infrastructure system transitions would imply, for example, changes in land and urban planning practices, together with deeper emissions reductions in transport and buildings.
4. Land use: Depending on the climate change mitigation objectives, land can be used as agricultural land for energy crops, pastures can be transformed towards forests or other possible changes of use, but this would pose profound challenges for a sustainable management of the various demands.

5. What could be the role of carbon dioxide (re)capture technologies?

All pathways that consider to limit global warming to 1.5°C by 2100, with limited or no overshoot, envision the carbon dioxide recapture of exceeding carbon emissions in any shape or form. In principle, removal (CDR), or carbon capture, to in most cases, such capture would compensate for residual emissions and contribute to achieve net negative emissions in case of an overshoot of global warming over 1.5°C.

Concretely, while it should be on the order of 100–1000 Gt CO₂ over the 21st century, the present potential of carbon capture, by mid-century would be limited to remove only about 9 Gt CO₂/yr. Reversing warming after an overshoot of 0.2°C or larger during this century would thus require its upscaling and deployment at rates and volumes that might not be achievable, given the considerable implementation challenges.

Existing and potential CDR measures differ indeed widely in terms of maturity, potentials, costs, risks, co-benefits and trade-offs. These include a series of options among which:

• Afforestation and reforestation, land restoration and soil carbon sequestration;
• Bio-energy with carbon capture and storage (BECCS);
• Direct air carbon capture and storage (DACCS);
• Ocean alkalinization.