Geoengineering, climate engineering, climate intervention or climate altering technologies - even if new terms are constantly being found, they do not change the fact that has been known for decades: climate modification approaches harbor major risks for environment and society.

Table of Contents

Geoengineering comprises deliberate and targeted interventions in the climate system - usually on a large scale - with the aim of mitigating man-made (anthropogenic) global warming (Royal Society 2009).

The numerous ideas can be divided into two main categories:

  1. Approaches to manipulate the earth radiation budget, called Solar Radiation Modification (SRM) and
  2. Approaches to remove carbon dioxide from the atmosphere, called Carbon Dioxide Removal (CDR).

Solar Radiation Modification (SRM)

Solar Radiation Modification (SRM) approaches aim to reduce the amount of solar radiation reaching the earth and thus reduce the global average temperature.

SRM would not eliminate the cause of climate change, but merely address global warming as a symptom. Ocean acidification, for example, would continue, as the concentration of CO2 in the atmosphere would remain the same or continue to rise. This also explains the greatest danger posed by SRM, the so-called termination shock. If the use of SRM were to be terminated at high CO2 concentrations, global warming would skyrocket, which could make it difficult or even impossible for humans, animals and plants to adapt. SRM would therefore have to be maintained either permanently or for many decades or rather centuries until the concentration of greenhouse gases in the atmosphere has decreased.

As SRM would modify solar radiation, it only works on the side of the Earth where it is daytime. In contrast, greenhouse gases are spread across the entire planet at all times of the day, preventing heat radiation from reaching outer space. SRM would therefore only ever have a limited area of effect. This one-sided manipulation of the Earth's radiation budget would have a strong influence on global precipitation patterns, especially the monsoon, and thus also on food security. It is also important to bear in mind that the cooling potential only relates to the average temperature. However, the cooling effect would not occur evenly due to the complex climate system: Warming would continue in one region, while overcooling would occur elsewhere.

How exactly these changes could be distributed across the Earth is currently being researched using computer models. However, these always contain a certain degree of uncertainty, so it will never be entirely clear how exactly the climate would change on a regional and short-term scale. Although the computer models could be improved with data from field experiments, field experiments do not provide conclusive certainty. Small-scale field experiments do not allow sufficient conclusions to be drawn about the consequences of a global deployment of SRM. And large-scale field experiments could already have similar risks to actual deployment. Due to the complex interactions within the climate system and with ecosystems, there are many uncertainties, most of which will remain unknown. The effects of SRM could, most likely, only be fully understood if it were tried out. Only one thing is already certain: the original climate could not be recreated. A completely new and unpredictable climate would emerge.

The approaches to influence the radiation budget can be divided into five categories:

Space Based Installations

Space mirrors that protect the earth from the sun - What sounds like "science fiction" at first is being seriously discussed, at least by some researchers. The idea is to place a huge solar sail or sunshade at a specific point between the earth and the sun. As this would require hundreds of thousands of rocket launches over several decades, researchers are considering how the resources for such a solar shield could be taken from space itself.

Stratospheric Aersosol Injection (SAI)

Probably the most widely discussed method is the introduction of substances into the stratosphere using specially designed aircraft*. Based on a large volcanic eruption, the aerosols deployed are intended to reflect the incident sunlight back into space and thus achieve a cooling effect. Due to the rough similarity to volcanic eruptions, the effects of SAI are still best researched. It is clear for example, that the chemical reactions would lead to a warming of the stratosphere and damage the ozone layer (see study "Geo-Engineering", german). This is particularly the case with sulphate aerosols. For this reason, the effects of calcite or diamond dust for example are also calculated in computer models. Larger field experiments with balloons have been prevented by the public in the past. However, there is an American company that regularly releases small balloons with sulphur dioxide and sells certificates for this alleged cooling effect.

Cirrus Cloud Thinning (CCT)

By thinning out high cirrus clouds, more infrared radiation should be able to escape into space instead of reducing the incident solar radiation. Nevertheless, this approach is discussed under the term SRM. By introducing particles as additional crystallization nuclei, several large ice crystals are to be created instead of many smaller ones, so that more heat radiation could escape through the optically thinner clouds. However, there is a risk that it could also have the opposite effect. Overall, there are considerable uncertainties regarding the formation and effect of cirrus clouds. Still, this influence on the radiation budget and its potential effect has been taken into account in a few computer models.

Marine Cloud Brightening (MCB)

The brightening or generation of stratocumulus clouds over the sea is to be achieved by introducing salt particles into the lower atmosphere, for example by spraying seawater from a ship with special vaporizers. Water from the air would then condense on the salt particles from the seawater or other salts and clouds would form. The solar radiation would then be reflected on the light-colored upper surface of the clouds. MCB is already being tested in small-scale field experiments, for example over Australian coral reefs, in order to achieve local cooling effects.

Increased Surface Albedo

Albedo refers to the ratio of reflected to incident solar radiation. Light-colored surfaces (e.g. snow) reflect more radiation and therefore do not heat up as much as dark surfaces (e.g. oceans, asphalt). This property can be used for cooling by painting roofs white, placing glass beads on icy surfaces or setting up reflectors in the desert. So-called "microbubbles" could be used to make the white foam of ships more stable. Genetically modified crops with lighter-colored leaves are also being discussed. As specific as the ideas are, the level of development and the environmental risks are just as varied.

UBA position on SRM

Due to the enormous risks and only temporary effect, the UBA rejects the use of SRM and, based on current knowledge, does not consider SRM as a future emergency option. Interfering with the Earth's radiation budget is no substitute for climate protection!

(See also: Geoengineering-Governance)


Carbon Dioxide Removal (CDR)

The aim of the approaches summarized under "Carbon Dioxide Removal (CDR)" is to reduce the concentration of the greenhouse gas carbon dioxide (CO2) in the atmosphere. This is to be achieved by "retrieving" the CO2 emitted and, at best, permanently removing it from the carbon cycle.

Plants remove CO2 from the atmosphere during photosynthesis and bind carbon in biomass. However, CO2 is released again when the biomass is decomposed or burned. Some CDR approaches aim to utilize the natural process of carbon sequestration. Other approaches attempt to technically imitate these natural processes with the help of chemicals and high energy input.

The transitions between natural and technical CDR are fluid and are categorized differently depending on the context. Another classification is based on whether the CDR method takes place on land or in the sea:

Marine CDR methods

  • Blue Carbon (Coastal ecosystem restoration)
  • Ocean Fertilization
  • Ocean Alkalinization
  • Artificial Upwelling, Downwelling
  • Direct Ocean Electrochemical Capture
  • Dumping of harvest waste and algae farms

See UBA topic - Marine Geoengineering (german)

Terrestrial CDR methods

  • Afforestation/reforestation
  • Wetland restoration
  • Agroforestry
  • Improved CO2-sequestration in agricultural soils
  • Biochar
  • Enhanced weathering
  • Bioenergy witht Carbon Capture and Storage (BECCS)
  • Direct Air Carbon Capture and Storage (DACCS)

See UBA topic Technical sinks (german) and Study on Nature based solutions for global climate action.

UBA position on CDR

UBA considers it risky to rely on partly unexplored and untested CO2

removal and storage technologies. The potential for CDR is however limited and can’t be determined solely based on technical factors. The social, economic and environmental sustainability must be addressed when assessing the potential. Natural sinks offer the possibility to remove a limited yet significant amount of CO2 from the atmosphere. CDR measures can’t replace a comprehensive reduction of greenhouse gas emissions (See UBA position on CCS and technical sinks and UBA brief position on CDR.


Geoengineering Governance

How precautionary European and international regulation should be designed in view of the problems and risks mentioned above is discussed here:

UBA topic Geoengineering governance


* To understand the distinction from so-called "chemtrails", see background paper (german)

Printer-friendly version
 geoengineering  Solar Geoengineering