"Geoengineering" or "climate engineering" may be defined as the deliberate large-scale manipulation of climate. The study of such techniques has been proposed in order to prepare for the cases that attempts to limit projected climate change by the reduction of greenhouse gas concentrations may fail or that consequences of climate change may prove worse than expected. Geoengineering may result in undesirable side effects for crucial parts of the Earth system and humankind.  Therefore it is necessary to study the possible efficiency, risks and implications. Numerical models are the only possible tool for performing initial scientific studies. Complex climate models will be used to quantify the effectiveness and side effects of such geoengineering concepts aiming at a reduction of the incoming solar radiation.

In general two main classes of geoengineering techniques are distinguished:

1) Carbon dioxide removal techniques (CDR) address the root cause of climate change by removing CO2 from the atmosphere: e.g. technical capture of carbon from the atmosphere. ocean fertilization to enhance uptake by biological systems, increasing the alcalinity of the ocean or afforestation on land.

2) Solar Radiation Management techniques (SRM) reflect sun light and heat back into space. They attempt to offset effects of increased greenhouse gas concentrations by causing the Earth to absorb less solar radiation. We have performed computer simulations to study the climatic impact of different approaches:

  • Enhancement of marine cloud reflectivity;
  • Placing shields or deflectors in space to reduce the amount of solar energy reaching the Earth;
  • Mimicking the effects of volcanic eruptions by injecting sulfate aerosols into the lower stratosphere.
  • Manipulation of cirrus clouds

Comparing climate impact of different geoengineering methods

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Geoenginering techniques have been proposed in order to prepare for the case that attempts to limit projected climate change by the reduction of greenhouse gas concentrations may fail or that consequences of climate change may prove worse than expected. As a geoengineered climate also differs from a natural climate of the same global mean temperature, studies on possible risk and side effects for crucial parts of the Earth system and humankind are necessary. Different geoengineering techniques of reducing incoming solar radiation, solar radiation management (SRM), have been proposed to possibly counteract global warming. Among them are the injection of sulfur into the stratosphere, mirrors in space, and marine cloud brightening through artificial emissions of sea salt.


Using the Earth System Model of the Max Planck Institute, scientists around Dr Ulrike Niemeier and Dr Hauke Schmidt of the department 'Atmosphere in the earth system' focused on the questions to what extent climate impacts of these three methods would be different. In their simulations the global forcing from the increase of greenhouse gases in a transient scenario (RCP4.5) was balanced over 50 years by SRM. While global mean temperature evolves similar with time for the different SRM methods, responses of global mean  precipitation and regional patterns differ considerably among the methods.  In all three SRM methods global precipitation is decreased compared to a scenario which keeps emission constant to 2020 level, most prominently for the aerosol based techniques: sea salt emissions and injection of sulfate into the stratosphere.


Niemeier, U., H. Schmidt, K. Alterskjær and J.E. Kristjánsson: Solar irradiance reduction via climate engineering – Impact of different techniques on the energy balance and the hydrological cycle. Journal of Geophysical Research-Atmospheres, doi:10.1002/2013JD020445, 2013.

Balancing CO2 increase via solar constant reduction: model simulation

Four state-of-the-art Earth system models simulate the impact of climate engineering.  The radiative forcing from an instantaneous quadrupling of the CO2 concentration (4xCO2), starting from the preindustrial level, is balanced by a reduction of the solar constant. This experiment mimics mirrors in space and reduces the incoming solar radiation at the surface. Model responses to the two counteracting forcings in G1 are compared to the control state, the preindustrial climate. While the global mean surface air temperature in G1 remains almost unchanged compared to the control simulation, the temperature is decreased in the tropics and rises in polar regions. Therefore, the equator to pole temperature gradient is reduced in all models. Another robust response is the global reduction of precipitation with strong effects in particular over North and South America and northern Eurasia. These robust resolts of the model ensemble show that an engineered climate would not be the same as natural conditions, mainly because of the reduced solar radiation at surface.

Schmidt, H. et al.: Solar irradiance reduction to counteract radiative forcing from a quadrupling of CO2: climate responses simulated by four earth system models, Earth Syst. Dynam., 3, 63-78, doi:10.5194/esd-3-63-2012, 2012.