The science of climate change

(updated 20 October 2020)

This blogpost addresses the science of climate change, and look at its legal implications. We will summarize the main facts, look at how scientists communicate their insights to the political sphere and to the public, and in how far the science of climate change is relevant from a legal perspective.

Overview

Today, climate change is studied by numerous scientists in a broad spectrum of disciplines, ranging from climatology to the social sciences. A broad consensus on the scientific insights on climate change is recorded in the IPCC Assessment Reports. The IPCC (“Intergovernmental Panel on Climate Change”) was set up in 1988 within the UN framework, and issues the so-called “Assessment Reports”. The latest of these reports, termed “AR5” (as it is the fifth Assessment Report), dates from 2014, and will be discussed in this blogpost.

The IPCC is currently preparing AR6, due to be published in 2021/2022. Additionally, it issues special reports, such as this 2018 report, parts of which you were assigned to read for the upcoming class.

The AR5 states at its beginning:

Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems. (AR5, SPM 1).

This sentence contains three significant statements:

1. climate change happens ( → rejecting claims that evidence of climate change is inconclusive)
2. climate change is mainly caused by humans, via the emission of greenhouse gases ( → rejecting claims that climate change has mainly natural causes)
3. climate change has a significant impact on both nature and humans ( → this is directed against claims that climate change would not have significant adverse effects)

What are the main drivers of climate change?

The main driver of climate change are increasing concentrations of greenhouse gases in the atmosphere. Greenhouse gases absorb and re-emit infrared radiation emitted by the Earth, and thereby warm up the atmosphere (= the so-called “greenhouse effect”). Other effects of increased concentrations of greenhouse gases in the atmosphere include ocean acidification (when CO2 is taken up by the oceans) and ozone depletion.

This graphic from the 5th IPCC Assessment Report shows the emissions of the different greenhouse gases by humans over the past 40 years (source: AR5, SPM p. 5)

The main greenhouse gases are:

• carbon dioxide (CO2) → CO2 is emitted when fossil fuels are burned for energy production (e.g. coal and gas plants) and transport (e.g. cars or airplanes), as well as in cement production and gas flaring (i.e., the burning of natural gas in fossil fuel production).
• methane (CH4) → methane is emitted in fossil fuel exploitation and by livestock (e.g. cows), among other sources
• nitrous oxide (N2O) → nitrous oxide (or laughing gas) is emitted through industrial processes, the burning of fossil fuels, as well as fertilization in agriculture
• Flourinated gases (F-Gases) → F-gases are used for cooling (refrigerators, air conditioning), sprays and foams.

This graphic shows total CO2 emissions since the beginning of industrialization. CO2 emissions increased drastically since 1945, and continued to rise even after a scientific consensus had formed that they contributed significantly to global warming (source: AR5, SPM p. 3)

The various greenhouse gases have different greenhouse effects, which are expressed as CO2-equivalents. The largest chunk of CO2 emissions stems from the burning of fossil fuels for energy production, transport and industry. Other significant emitters are cement production and gas flaring, as well as agriculture and other land use (e.g. de-forestation). The following graphic shows how the GHG emissions can be attributed to the different economic sectors.

“AFOLU” stands for Agriculture, Forestry and Other Land Use; “direct GHG emissions” are emissions that are directly emitted (e.g. emissions coming out of the factory chimney); “indirect emissions” are emissions from energy production, which can in turn be attributed to the various economic sectors (e.g. the electricity the factory consumes). (source: AR5, Synthesis Report, p. 47).

What are the main effects of climate change?

Observable effects of climate change on natural systems include:

• an increase in average temperature,
• a rise in sea levels,
• shrinking glaciers and ice sheets in Greenland and the Antarctic,
• an acidification of the oceans
• increase of extreme weather events (e.g. heat waves, droughts, floods, storms)

These graphics show the observable increase of global temperatures and the rising of sea levels. (source: AR5, SPM, p. 3)

How will climate change shape the 21st century?

Regarding the future development, the IPCC Assessment Report makes the following general statement:

Surface temperature is projected to rise over the 21st century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions. The ocean will continue to warm and acidify, and global mean sea level to rise. (AR5, SPM p. 10; “precipitation” = e.g. rain, snow or hail)

However, the future development path depends to a large extent on the total of CO2 emissions that remains in the atmosphere (thus, past and future emissions), which are termed “cumulative emissions”:

Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Projections of greenhouse gas emissions vary over a wide range, depending on both socio-economic development and climate policy.

The IPCC report provides different scenarios of future developments (or “pathways”), which differ in regard to how fast and deep GHG emissions are cut. You can see that in the different scenarios CO2 emissions “peak” (i.e., the highest level of emissions) in different years (2020, 2050, 2070, or not at all). The “business as usual” (or “baseline”) scenario assumes that no emission cuts are enacted, and that future GHG emissions will increase like in the past (see the “RCP8.5” scenario in the upper graphic on the left, indicated by a red line). According to current estimates, the “baseline” scenario may lead to a global temperature rise (above pre-industrial levels) of 4–5 degrees Celsius in 2100 (see grey circle in the lower graphic on the left). According to the “RCP4.5” scenario (light blue line), which would require a quick end to emission growth, and significant emission cuts from around 2050 onwards, the estimated temperature increase at the end of the century would still be around 2–3 degrees Celsius. However, the international community has defined as its target in the Paris Agreement (concluded in 2015) to stay “well beyond 2°C”:

“ […] Holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change;” (Paris Agreement, Article 2.1.a)

According to the IPCC Report, this would require deep emission cuts by 2030, and an end to CO2 emissions by 2070. The IPCC Report describes the risks associated with climate change as follows:

Climate change will amplify existing risks and create new risks for natural and human systems. (AR5, SPM p. 13)

In particular, the Report points out the following risks:

• largely increased extinction rate for many species
• threat to food security (fishing and agriculture)
• water scarcity
• increased health problems
• significant economic costs
• displacement of people

It is important to note that, according to the IPCC, climate change has uneven effects:

Risks are unevenly distributed and are generally greater for disadvantaged people and communities in countries at all levels of development. (AR5, SPM p. 13)

We will come back to this issue in the next class, which deals with the economic and political dimension of climate change.

Producing science …

While evidence that CO2 emissions could influence the climate date back more than a century, scientists have systematically studied this effect since the 1960s, with scientific evidence greatly solidifying in the 1970s. A well-recognized point of change was in 1988, when NASA scientist James Hansen presented scientific evidence to the US Congress. Today, the scientific knowledge on climate change is aggregated in the IPCC reports.

The different parts of the IPCC 5th Assessment Report: it consists of the Report 1 on the scientific basis, Report 2 on the impacts of climate change, and Report 3 on mitigation (i.e., preventing or limiting it).

If you look into the IPCC Assessment Report, you will see how complicated climate science is. Billions of tiny bits of information — collected from ice cores, temperature and other weather measurements at thousands of places, satellite images, etc. etc. — have been brought together in thousands of peer-reviewed articles over the past decades, slowly putting together the climate puzzle. This required the development of new measuring instruments, complex computer models and novel scientific methods, because the previously existing ones would only be of limited help to study the phenomenon of climate change.

This already shows that scientists do not just “discover” facts around climate change — they actively look for them. Climate science is thus a social process, or, in other words: human beings doing stuff together. They discuss, collaborate, quarrel, make mistakes, change their opinion, etc. This means that there will be disagreement or uncertainty on many of the details. At the same time, there is no disagreement on the central facts: multiple large-scale surveys have shown that virtually all climate scientists agree that climate change is in fact happening, and that it is caused by humans (one major study has found agreement among 97% of climate scientists, a number that you may have already encountered).

But how should scientists deal with this situation? Wait until every last fact became uncontroversial before they propose solutions? Given that climate science deals with the complex dynamics of a whole planet (including how humans interact with it) their knowledge can never be perfect. Consequently, climate scientists have to communicate provisional (“as far as we know …”) and conditional (“assuming that …”) knowledge to the public.

… and communicating it

Moreover, science is complicated, and neither politicians nor regular people have the ability or time to understand the thousands of articles that have been written, or would be able to decide which policies they should enact in reaction. Consequently, scientists have to find ways to communicate their insights to the public and to policymakers. This implies difficult choices: neither should their communications be to complicated (otherwise nobody would understand what to do) nor to simplistic (otherwise they would be accused of manipulating the public). They should not be too detailed (“there is a 54–63% chance that X may happen within the next 3–15 years, assuming that factors a,b,c,etc remain constant …”), but at the same time they should properly describe the complexity of the situation. Climate research (like any research) and its communication therefore imply difficult, ultimately “political” choices.

Moreover, scientists also have to react to the needs of the public and of policymakers, and produce science that actually is useful to somebody (this is called the “co-production” of knowledge). Knowing what has happened in the past is one thing, but knowing what might happen in the future another. And in regard to climate change, the latter is decisive for policymakers. However, climate scientists are no fortune tellers, the climate is a complex system, and the future behavior of humans cannot be known. Consequently, climate scientists have to make projections of future developments, but they can neither be too deterministic (“X will definitely happen in 101 years”), nor too vague (“we have found that 50.000 different climate scenarios are possible”). They have to provide a clear understanding of how human choices will affect the climate, but also cannot get around the fact that the future is always uncertain.

The complicated procedure leading to the IPCC assessment report. It can be seen that, apart from the scientists themselves, governments play an important role, in nominating scientists, reviewing the draft, and agreeing to the “Summary for Policymakers.” Beyond that, the IPCC bureau plays more than just an administrative role as well (graphic here)

Institutions such as the IPCC are therefore scientific as well as political actors. They aggregate scientific insights, but they also do it in a way that is actually useful for non-scientists. The double character of the IPCC is manifested, for example, in the fact that their report is compiled by scientists, but the “Summary for Policymakers” is put together on this basis by the representatives of the states. Thus, the IPCC structure is relevant in the production and communication of science (see for different proposals to reform the IPCC here).

Different ways of communicating the likely effects of climate change

Because science is complex and conditional to an extent that non-scientists cannot deal with in a useful way, scientists, activists and policymakers have been proposing different “standards”. Each of them have different advantages and disadvantages:

Facts and accessible communication at the same time: a graphic depiction of the global temperature increase over the past two centuries. See here for an animated gif.

• Limit of global temperature rise. The well-known number 2C stems from an economics paper by William Nordhaus from the 1970s. It has long been criticized for exceeding the limit that scientists in the past decades actually consider reasonably safe; however, it was increasingly taken up in policy and in public discourse; it was picked up in 2010 in the Cancun Agreements. The 2015 Paris Agreement aims at 1.5C maximum, and well below 2C (read a short history of the 2C number here).
• CO2 particles in the atmosphere. It is argued that temperature increase should not only be stopped, but that the amount of CO2 in the atmosphere must in fact be reduced in the future, namely to below a safe level of 350 parts per million (ppm)of CO2 in the atmosphere. The current level is 400ppm, and increases by 2ppm every year. The environmental NGO 350.org is named after this goal.
• Carbon budget. In order to depict how much more greenhouse gases can be emitted before the 2C threshold is reached, the idea of the carbon budget has been picked up, for example by the IPCC. It is argued that the total amount of emissions is 1 trillion tons of carbon. Until today, around half of the budget has been “spent”, and it will be fully used within the next three decades.

350.org, founded by Bill Mckibben, advocates for a reduction of atmospheric CO2 concentrations to below 350pp.

None of these indicators has remained uncontroversial. It has been argued, for example, that the 2C target depicts only part of the problem, and causes misunderstandings. For example, weather extremes caused by climate change may also increase snowfall and low temperatures at some places, which might be misunderstood by people as conflicting with the climate change hypothesis (this issue has often been picked up by climate change deniers). It has been proposed that a broader set of targets and indicators should instead by used (similar to the Millennium Development Goals to reduce global poverty). However, the 2C goal has been lauded for achieving the necessary simplification of the problem, making it comprehensible to policymakers and the general public. Another commentator compared the 2C limit to speed limits on the road:

“The level of danger at any particular speed depends on many factors… It would be too complicated and unworkable to set individual speed limits for individual circumstances taking into account all these factors, so clear and simple general speed limits are set using judgement and experience to try to get an overall balance between advantages and disadvantages of higher speeds for the community of road users as a whole.” (quoted in: Carbon Brief)

Given that it is the best-known indicator relating to the dangers of climate change, the 1.5C or 2C limit will stay around for the foreseeable future. Moreover, the 1.5/2C limit has become legally relevant, being enshrined in the Paris Agreement.

xkcd illustrating the future implications of the path of global temperature rise that we are currently on.

How is the climate science relevant from a legal perspective?

Obviously, science (hopefully) informs policymaking. However, this takes complex forms, because multiple political interests are at stake. Beyond that, climate science plays a role in legal processes in other ways, too:

• Most notably, this is the case in liability (i.e., “tort”) lawsuits. When large CO2 emitters are sued for damages caused by climate change, a court needs to establish causation: did the emission cause the damage? With climate change, the question is more difficult than in the regular liability lawsuits that judges usually get to see (think about car accidents). Even though CO2 emissions are known to cause climate change, did they also cause or aggravate the specific weather phenomenon that created the damage? And in how far can one emitter — even if it is a large one — be held accountable for damages caused (maybe even in another jurisdiction), if others have contributed as well? We will address these questions in a later class, which will deal with climate change tort lawsuits.
• Another example is the ground-breaking Urgenda case. Here, the Dutch NGO Urgenda sued the Dutch government for failing to implement emission cuts sufficiently. In deciding in favor of the NGO, the Court found the IPCC Reports to be the relevant basis to establish the Dutch government’s obligations.