Liu et al. (2013) investigated two aspects of precipitation change under climate change conditions: changes in precipitation levels and sea surface temperatures (SSTs). These two aspects, the change in global mean precipitation and the spatial distribution of precipitation changes, are found by Liu et al. to have different responses to increases in greenhouse gases from anthropogenic sources than to combined solar radiation and volcanic forcings. This supports the incongruity between the predictions of climate change reconstructions based on palaeoproxy evidence, naturally occurring records of climate conditions and time, and the current best climate models. While increased atmospheric and sea temperatures, as a result of a warming event, have historically caused an increase in precipitation due to an increase of energy in the atmosphere and therefore a more energetic system, the aerosols introduced during the production of greenhouse gases through the burning of fossil fuels suppress precipitation, leading to less overall precipitation in a greenhouse gas influenced environment. Similarly, the spatial distribution of precipitation is affected by the forms of warming, causing different gradients of SSTs. Several climate models predict that these gradients change differently under the solar-volcanic warming than greenhouse gas associated warming, again probably due to the presence of aerosols. When the findings from both the change in global mean precipitation and the change in spatial distribution of precipitation are considered, it becomes clear that the method of climate change is important in predicting the way that precipitation patterns will change.—Alison Marks
Liu, J., Wang, B., Cane, M., Yim, S., Lee. J., 2013. Divergent Global Precipitation Changes Induced by Natural Versus Anthropogenic Forcing. Nature 493, 656–659.
Jian Liu and his colleagues used the ECHO-G model to run millennium climate simulations. The ECHO-G is an atmosphere-ocean model that is able to reproduce accurate short-term and present-day climates by coupling an oceanic circulation model and an atmospheric model. Comparisons between this and other more complicated climate models found them to be relatively similar, adding a sense of validity to the present model.
Liu et al. found that the difference in global mean precipitation change between the two forms of climate change can be best explained through the tropospheric energy budget. The troposphere, the lowest layers of earth’s atmosphere, has a certain amount of energy available to do work. This energy is responsible for the warming and evaporation of water from the oceans, the formation of clouds, and other atmospheric activities. When more energy is put into this system, i.e. during global warming, more water should evaporate and more precipitation should fall. However, the release of anthropogenic greenhouse gases is accompanied by the release of aerosols, fine particles suspended in the air, which discourage cloud formation and precipitation. Therefore, climate change due to greenhouse gas release can have a net negative relationship with annual precipitation.
More involved modeling was used by Liu et al.to consider the possible differences between solar-volcanic and greenhouse gas climate change with regard to the distribution of precipitation. They began by identifying a solar-volcanic mode during the last millennium, the Medieval Warm Period (1100-1200 CE). This period was then compared, using both proxy evidence and climate modeling, to the Little Ice Age (1630-1730 CE) to determine the changes in precipitation and SST during the warm period. The Medieval Warm Period is characterized by a stronger SST gradient across the Pacific Ocean, stronger winds across the Pacific, stronger atmospheric circulation, and, as a result, higher precipitation. Liu et al. compared these results to climate modeling that only considered greenhouse gases as a climate change forcing, both for the periods 1860-2000 CE and 1990-2100 CE. The period between 1990-2100 CE was modeled using the A1B scheme proposed by the IPCC. In the A1B climate model, all energy forms are improved, but all forms, both carbon-neutral and not, continue to be used in equal amounts. Both time periods had similar SST and precipitation trends predicted by the model, however the results during the later time period were exacerbated to a greater degree. The later time period had a slightly smaller increase in SST gradient change than the solar-volcanic period and a comparison of precipitation changes shows that the two climate change forces caused precipitation change in different regions. While the solar-volcanic model was drier than the greenhouse gas model in the central Pacific, the greenhouse gas model was drier along the tropics. Overall, the greenhouse gas model creates about 40% less precipitation change per degree Celsius of temperature change than the solar-volcanic model.
The research done by Liu et al. helps to explain the complicated nature of climate change. The source causing the climate change can greatly impact the oceanic SSTs and precipitation patterns. Their work also makes it clear that while palaeoproxy data can be useful in helping to ascertain a general idea of the future through comparison, some of the aspects of the climate change will not follow the same pattern as past warming events.