contact:Ulrike Niemeier, Hauke Schmidt

Climate engineering" or “geoengineering” 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 emissions may fail.

In general, two main categories of geoengineering techniques are distinguished:

1) Carbon dioxide removal techniques (CDR) would address the root cause of climate change by removing greenhouse gases from the atmosphere.

2) Solar Radiation Management techniques (SRM) would attempt to offset effects of increased greenhouse gas concentrations by reducing the amount of sunlight absorbed by the earth.

Effectiveness and side effects of the proposed techniques are still largely uncertain. Therefore, our research focuses on a better understanding of effectiveness and risks of SRM techniques (among them the suggestion to mimic the effect of large volcanic eruptions by artificially injecting sulfur into the stratosphere) and in particular the question what climate they would produce.

To this end we are using numerical climate models, often in the framework of model intercomparison projects. We have coordinated EU FP7 project IMPLICC, where several European groups performed a coordinated set of numerical experiments, and are participating in the Geoengineering Model Intercomparison project (GeoMIP). We have been involved in the European Transdisciplinary Assessment of Climate Engineering (EUTRACE), and are participating in two projects of the priority program 'Climate engineering: Risk, challenges and opportunities' of the German Science Foundation (DFG).

A focus of our research can be summarized in the question 'Can injections of sulfur into the stratosphere help to counteract climate change?' (Niemeier and Tilmes, 2017). This Science Perspective article provides an overview of current knowledge of side effects and related governance issues. Stratospheric aerosol modification (SAM) techniques are presently not developed and knowledge of impacts is based on model studies only.
Major results of our research are related to the effectiveness of sulfur emissions into the stratosphere (Niemeier et al., 2012). It has become clear that the cooling efficiency, the cooling per injected unit of emitted sulfur mass, decreases with increasing injection rate (Niemeier and Timmreck, 2015). Thus, the more SAM is done, the less effective further injections are at reducing temperatures. The impact of the sulfate heating in the stratosphere intensifies this effect by influencing dynamics and feeding back on the sulfate distribution (Niemeier and Schmidt, 2017).

A robust pattern of climate responses to SRM is the slowing of the hydrological cycle. An engineered climate would be different to a natural climate, especially with reduced global mean precipitation (Schmidt et al., 2012). However, the specifics of the response of the global hydrological cycle to SRM depend on the technique (Niemeier et al., 2013).

Selected publications:

Niemeier Ulrike and Simone Tilmes, Sulfur injections for a cooler planet, Science, Vol. 357, Issue 6348, pp 246-248, 2017. (Invited paper)

Niemeier, U. and Schmidt, H.: Changing transport processes in the stratosphere by radiative heating of sulfate aerosols, Atmos. Chem. Phys. Discuss., doi.org/10.5194/acp-2017-470, in review, 2017.

Niemeier, U., Schmidt, H., Alterskjaer, K., & Kristjánsson, J. E., Solar irradiance reduction via climate engineering: Impact of different techniques on the energy balance and the hydrological cycle. Journal of Geophysical Research-Atmospheres, 118, 11905-11917, (selected as AGU spotlight), 2013.

Niemeier, U., Schmidt, H., & Timmreck, C., The dependency of geoengineered sulfate aerosol on the emission strategy. Atmospheric Science Letters, 12, 189-194, 2011.

Schmidt, H., Alterskjaer, K., Bou Karam, D., Boucher, O., Jones, A., Kristjansson, J. E., Niemeier, U., Schulz, M., Aaheim, A., Benduhn, F., Lawrence, M., & Timmreck, C., Solar irradiance reduction to counteract radiative forcing from a quadrupling of CO2: climate responses simulated by four earth system models. Earth System Dynamics, 3, (see EGU press release), 63-78, 2012.

Further publications can be found on the GeoMIP web page.