3.2. Study of the Earth system

Research on the Earth system has developed rapidly since the 1980s. This development has been driven by an understanding of the increasing impact of human activity and the potential for sudden and irreversible changes in the functioning of the Earth system. This research has been aided in particular by the development of observational methods and modelling, and by the extensive scientific syntheses and assessment reports that have been compiled from the data.

One example of an important method of observation is the measurement of atmospheric carbon dioxide levels at the Mauna Loa Observatory in Hawaii, which has been ongoing since 1958. The measurement shows an unquestionable increase in atmospheric carbon dioxide levels. Today, satellites can observe climate variables, atmospheric composition, vegetation changes, ocean properties, urban development, etc. with high spatial resolution, providing a near real-time picture of the state and functioning of the Earth system. The understanding of the history of the Earth system has also improved. For example, analysis of ice samples from the Antarctic ice sheet has provided information on past climate variability and related variables, which can be used to predict future changes in the Earth system.

The functioning of the Earth system can nowadays be modelled using circulation models based on basic principles from physics and chemistry, which take into account the interactions between the atmosphere, the land surface, the oceans and the biosphere. These models can also incorporate anthropogenic emissions of both fine particles (aerosols) and greenhouse gases to try to identify possible future climate trends. However, the ability of these models to predict the behaviour of the system must be viewed with caution, as there is considerable uncertainty in their parameters (i.e. the numerical values describing the behaviour of the system) and the models do not capture all possible interactions. 

There are also integrated assessment models that have been developed to examine the impacts of alternative courses of action on both the economy and the climate, but these models should also be viewed with caution in terms of their ability to predict the future. While these models may not provide an accurate picture of reality, they are valuable tools for the integration of a wide range of processes. The ability of models to describe the functioning of the Earth system can be tested by comparing their predictions with historical observations.

Over the past two decades, several major assessment reports and syntheses have been produced, bringing together research on the Earth system. Of particular importance have been the many assessment reports of the International Panel on Climate Change (IPCC) and the Intergovernmental Panel on Biodiversity and Ecosystem Services (IPBES). These panels and reports operate at the interface between policy-making and research, communicating research findings to policy-makers and opening up new research directions based on feedback from the policy sector.

Tipping elements and cascading effects

Anthropogenic pressures have also raised concerns about potential tipping points, which refer to sudden and potentially irreversible changes in the functioning of a system as a result of increased pressures. Concerns have been raised about certain elements of the Earth system that are thought to be particularly vulnerable in a warming climate. These parts of the Earth system have been referred to as 'tipping elements'. A characteristic of tipping elements is that once a change has been set in motion, it may be impossible to stop it. For example, in the Arctic Ocean, the reduction in ice cover leads to accelerated warming, as the sun heats open water much more efficiently than water covered by ice sheets.

The figure below shows possible tipping elements that are sensitive to global warming and the links between them. The Greenland ice sheet, the summer ice cover of the Arctic Ocean, the Alpine glaciers, coral reefs and the glaciers of western Antarctica are already at risk of disappearing as a result of current global warming. The melting of the Greenland and western Antarctic glaciers would lead to a rise in sea levels of several metres in the next few centuries at the shortest.

Figure. Possible tipping elements of the Earth system and the connections between them that can lead to cascading chain reactions. Elements already at risk from current global warming are shown in yellow, elements at risk from a warming of 3-5 degrees Celsius are shown in orange, and elements at risk from a warming of more than 5 degrees Celsius are shown in red. Source Steffen et al. (2018).

If the climate warms by three to five degrees Celsius compared to pre-industrial times, at risk of disappearing or changing are northern coniferous forests, jet streams, the circulation of surface and deep ocean water, the Amazon rainforest, the periodic changes in ocean currents in the Pacific (making the El Niño phenomenon permanent) and the Indian summer monsoon, among others. The climate in the Sahel region is also projected to change, but models give mixed results on the nature of the change. If the climate warms by more than 5°C, the Arctic Ocean could become completely ice-free and the permafrost in Siberia and the glaciers in eastern Antarctica will start to melt.

Links between the tipping elements mean that changes in one element increase the likelihood of change in the linked elements. Such a chain reaction would increase the likelihood of catastrophic planet-wide changes at much lower temperatures than an assessment of individual elements would suggest. However, interactions between different elements are difficult to study, so the corresponding risks are difficult to assess with confidence. Furthermore, we do not know whether all the elements involved have even been identified. For example, very recent studies show that the formation of stratus clouds, which cool the Earth, ceases when the atmospheric carbon dioxide concentration exceeds a certain limit (1200 ppm). Without these clouds, the Earth's temperature rises by about eight degrees Celsius. This change would be very persistent, as the re-formation of the clouds requires a drop in CO₂ levels well below the level at which cloud formation was initially disrupted.

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