Earth’s energy balance and the physics of the greenhouse effect provide scientific explanations for human-made climate change. The greenhouse effect occurs when trace gases in the atmosphere react to radiation and change the planet’s energy balance. Without this natural effect, the earth’s average surface temperature would be around minus 18 °C, instead of 14 °C, thus making life on Earth impossible.

How does the greenhouse effect happen?

The majority of short-wave solar radiation hitting Earth is beamed back from Earth’s surface in the form of long-wave infrared radiation. However, cloud water vapour and the greenhouse gases carbon dioxide and methane absorb some of this long-wave counterradiation and emit it in random directions. In this way they prevent heat radiation entering space unhindered.

If the number of molecules of greenhouse gases increase in the atmosphere, the counterradiation displaces into greater heights in the atmosphere. Due to the colder temperatures there, less heat radiation returns to space, which leads to an excess of energy in the lower atmosphere. The results are higher temperatures on earth’s surface and in the lower atmosphere.

Gases such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) are only present in the per mille range in the atmosphere. The percentage of CO₂ for example is 0.04 percent. If this gas concentration increases from a few particles per million or billion particles (parts per million/ppm or parts per billion/ppb) it can have significant effects. CO₂ concentration was approximately 280 ppm in the pre-industrial era. In 2021 it reached, according to information from the US weather authority NOAA, a new annual highest value of almost 415 ppm. Even if it seems small at first glance, the increase has a measurable impact due to the gases’ greenhouse effect. It has caused the global average temperature to rise by 1.2 °C.
 

What role do aerosols play?

Aerosols are tiny soot or dust particles in the atmosphere in the micro or nanometre range (millionth to billionth of a metre). They come from natural sources such as volcanoes or sandstorms, but are also released in large part due to humans burning natural oil and coal.

Aerosols alter the energy balance because they reflect short-wave solar radiation. The higher the aerosol concentration in the atmosphere, the less solar radiation reaches the earth’s surface. Aerosols thus have a cooling effect. If we observe the effect of greenhouse gases independently of other influences, a warming by some 1.5 °C would have taken place since the beginning of the 20th century. Yet, due to the effect of aerosols, there is only an increase of a little more than 1 °C.

Particularly following World War Two, intensive residual burning of natural oil and coal took place which carried many sulphur aerosols into the atmosphere. The light temperature increase stagnated in the 1970s, primarily due to the cooling effect of air pollution. The sulphur content of these aerosols was massively reduced when flue gas cleaning systems were installed in modern coal plants. This reduction led to an accelerated temperature increase.

Volcanoes are one of the biggest natural sources of aerosols. They primarily release sulphur aerosols that are active in the atmosphere for a limited time – a few months to a year – and are then washed out again. Many of these substances enter the lower stratosphere where exchange with the weather-active troposphere below takes time. After the troposphere the stratosphere forms, so to speak, the second storey of the earth’s atmosphere and extends from some 12 kilometres above the earth’s surface up to a height of 50 kilometres.

How accurate are climate models?

Computer models help project the future climate within certain limits. Researchers are using these climate models to try and calculate climate scenarios based on specific assumptions for various factors. The focus is on various development paths based on specific assumptions, not concrete predictions.

The models are based on base equations of physics and chemistry laws (for example energy conservation, mass and impulses), which are then applied to situations of the atmosphere, the oceans and the earth’s surface. They can be combined with ice models of the cryosphere as well as biosphere vegetation models. In this way it is possible to describe interaction processes between the Earth System components and to use models for past and future climate situations.

Gains in knowledge and more powerful computers have facilitated the models’ further development over the decades. Yet even the first, comparatively simple global climate models around thirty years ago were good enough to predict the increase of the global average temperature in specific greenhouse gas scenarios.

Development of the models is initially limited to individual areas such as the ocean, land mass or the atmosphere, since the interactions between these spheres are especially complex. These individual components are then added to a coupled climate model. Developments like these take many years. This why researchers refer to generations of climate models. Research is currently focused on Generation 6 climate models, or CMIP6, which stands for Coupled Model Intercomparison Project Phase 6.

Continual further developments mean that now it is not only possible to predict the temperature but also to make more reliable advance calculations regarding other parameters such as changes to rainfall and the water cycle. While projections were initially only possible on a global level, they can now also be carried out for individual countries and regions.

Ongoing testing of all factors and assumptions is part of the further development of climate models. It must be possible to measure theoretical models against reality. Incidental events are also sometimes helpful, as they can be considered as natural experiments. Climate modelling has shown that the increasing temperatures in the atmosphere can be traced back to the CO₂ emissions caused by people.
 

Palaeoclimatology: What does looking back reveal?

Meaningful statements about changes are only possible when the climate can be observed over a long time period. Regular and comprehensive weather records, however, have only existed since the mid-19th century and the concentration of CO₂ in the atmosphere has only been continually documented since the 1950s. To now identify data about this comparatively short time period we need to use palaeoclimatological tools. They attempt to identify the changes of the climate in the course of Earth’s history so as to be able to compare today’s developments with those of the past. It is only thanks to the results of paleoclimatology and the comparison with the past that current climate change can be classified as an extraordinary phenomenon caused by humans and not as a natural deviation that could take place again of its own accord.

Researchers refer to “climate archives” for their research. These are natural eyewitnesses such as tree rings which allow us to reconstruct the climate of the past. Ice bores from the depths of the Arctic and Antarctic ice sheets represent two of the most important archives. With every additional metre from the depths of compressed snow, researchers can analyse parameters that go back further and further in time. So far it has been possible, for example, to define the content of greenhouse gases for the past 800,000 years based on the air bubbles trapped in the ice. Current bores in Antarctica are now working on unearthing ice up to 1.5 million years old.

Land and ocean sediment is investigated for an even further look into the past. Researchers observe growth layers of coral reefs or tree rings when it comes to more recent time periods. Assigning plant communities and animal species to certain geological time periods also helps when defining climate conditions. Thanks to this combination of various sources, it is now possible to trace known climate history back, almost completely, by approximately 100 million years.

Scientists can use this data to check hypotheses about the climate of earlier times as well as to better understand the changes observed in the climate system today. One thing becomes clear: The current situation is unprecedented in the history of humanity. Since humans have been on Earth the temperature has never risen as fast as it is doing now.

What is the significance of tipping points?

Alterations caused by climate change can take place both continuously and gradually and would be reversible, at least in principle. However, should they reach critical thresholds it can result in sudden and very fast changes that are no longer stoppable and could even accelerate global warming. These “tipping points” are critical parts of the Earth System which can irreversibly change if they are submitted to too much pressure under global warming. They act like a cup that falls off the table and smashes or a boat that rocks on the water and capsizes.

The ice sheets in Greenland and the Antarctic, important ecosystems such as the Amazon rainforest, or ocean current systems such as the Atlantic Ocean Circulation and the Gulf Stream, or the Indian summer monsoon all represent tipping elements in the climate system. If they reach tipping points, it could have self-amplifying and accelerating effects. Enormous ice areas such as the West Antarctic ice sheet could completely vanish without any hope of replacement and drastically raise sea levels. Or if the Gulf Stream were to slow down, Europe’s climate could cool while other regions heat up.

Another example is Siberia’s permafrost region, which is seeing increasing exposure to heatwaves. The upper ground level thaws in the summer season for longer and deeper and releases more and more methane and carbon dioxide. The climate gases then propel the greenhouse gas effect and further stimulate global warming. The thawing of the permafrost ground also endangers the stability of cities, transport routes, pipelines and industrial facilities.

Is the Gulf Stream on the verge of collapsing?

The most significant tipping points of the climate system also include the global ocean currents. Continual balancing currents in the atmosphere and the oceans exist because the sun heats up the planet to varying degrees. One of these large marine movements is the Atlantic Ocean Circulation, to which the Gulf Stream and the connecting North Atlantic Current belong. These currents transport warmth to the western side of Europe and ensure that there is an overall milder climate in Europe than regions in comparative latitudes, such as Canada, for example. Differences in temperature and salt content of the sea regions keep the current going. After water has cooled in the northern latitudes it sinks to great depths, absorbs large amounts of carbon dioxide from the atmosphere – and thus becomes one of the earth’s most important carbon sinks.

Climate change could weaken the Atlantic Ocean Circulation, for example via the increased flow of freshwater as a result of melting Greenland ice or via more rainfall. Observations and models show that the circulation, especially the Gulf Stream, has indeed weakened in the last decades. The extent of this is, however, the subject of current research. Calculations from a few years ago showed that it was already 30 percent weaker, but this was later corrected to around ten percent. As yet it has not been concluded whether the weakening can be linked to the consequences of global warming. A sudden collapse of the Gulf Stream is not a likely scenario, at least not before 2050.

How much CO₂ can still be emitted?

The strong link between CO₂ concentration in the atmosphere and the global temperature means it is easy to estimate the maximum amount of greenhouse gas humanity can still release without exceeding a certain global temperature increase. Scientists call this a “carbon budget”. That said, it is not possible to calculate an exact amount for a specific time period before a specific limit such as the 2 °C target is reached. This means that a probability is always involved when speaking about the carbon budget.

According to the sixth IPCC report, there is a 66 percent probability that humanity will not exceed the 2 °C limit provided that it releases a maximum of a further 1,150 gigatonnes (billion tonnes) of CO₂. The Carbon Clock at the Mercator Research Institute on Global Commons and Climate Change indicates that the budget for staying below the 2 °C threshold will be exhausted in around 25 years (as of August 2022) at a constant annual emission of 42.2 gigatonnes. Only seven years remain for the 1.5 °C goal. Should, however, 1,350 gigatonnes be emitted in total, the probability of missing the target of maximum 2 °C increases to 50 percent.

How resilient is Earth?

Climate change is just one of several global issues presenting a challenge to humanity. In 2009, the working group of the Swedish resilience researcher Johan Rockström introduced the concept of “planetary boundaries”. These are boundaries that must be adhered to in order not to endanger the stability of global ecosystems and the living conditions of humans.

The nine planetary boundaries below are usually those considered to offer safe leeway for Earth to remain stable and its inhabitants to live safely and sustainably.

• Climate change
• Condition of the biosphere
• Freshwater
• Ocean acidification
• Ozone layer
• Air pollution
• Land use
• Material cycle (nitrogen and phosphorous)
• New substances (chemicals and plastics)

In 2009 three of the nine boundaries or at least parts of them had already been exceeded – namely climate change, loss of biodiversity and the nitrogen cycle. The boundaries of phosphorous cycle, land use, and chemicals and plastics have now also been exceeded. In terms of freshwater it is clear that the “green water” available to plants has entered the risky zone. The concept of planetary boundaries emphasises how climate change is not an isolated concept. Instead it is part of an array of connected and interactive changes in the Earth System.

Is there a risk of a heat age?

Since the Paris Agreement, the global community is aiming to limit the maximum temperature increase to between 1.5 and 2 °C in order to keep the damages and risks somewhat manageable. Carbon-neutral life must succeed worldwide by 2050 for this target to be achieved. It seems increasingly uncertain as to whether this will happen. This is why climate researchers calculate further scenarios for the event that the measures to limit greenhouse gas emissions are not sufficient. In these projections, temperatures increase well beyond the target of limiting warming to 2 °C. Instead, they are based on a warming of 4 °C to 5 °C, known as “Hothouse Earth”. Without reinforcement of the political measures extending beyond those introduced at the end of 2020, we could see a global average warming of 3.2 °C by 2100.
 

According to the sixth IPCC report the global surface temperature will increase in all observed emission scenarios until at least the middle of the century. Global warming of 1.5 °C and 2 °C could be exceeded over the course of the 21st century – unless drastic reductions in carbon dioxide and other greenhouse gases are achieved in the next decades.

Yet even if we do succeed in stopping the temperature increase at approximately 2 °C, it cannot currently be ruled out that some tipping points such as coral reefs, permafrost ground, ice in Greenland and the ocean, or the glaciers would see feedback effects that could in turn trigger cascades and unstoppable processes. This would then cause an increase in greenhouse gases despite countermeasures and without human contribution.

Published: August 2022