Robust direct effect of carbon dioxide on tropical circulation and regional precipitation

Abstract

International audiencePredicting the response of tropical rainfall to climate change remains a challenge(1). Rising concentrations of carbon dioxide are expected to affect the hydrological cycle through increases in global mean temperature and the water vapour content of the atmosphere(2-4). However, regional precipitation changes also closely depend on the atmospheric circulation, which is expected to weaken in a warmer world(4-6). Here, we assess the effect of a rise in atmospheric carbon dioxide concentrations on tropical circulation and precipitation by analysing results from a suite of simulations from multiple state-of-the-art climate models, and an operational numerical weather prediction model. In a scenario in which humans continue to use fossil fuels unabated, about half the tropical circulation change projected by the end of the twenty-first century, and consequently a large fraction of the regional precipitation change, is independent of global surfacewarming. Instead, these robust circulation and precipitation changes are a consequence of the weaker net radiative cooling of the atmosphere associated with higher atmospheric carbon dioxide levels, which affects the strength of atmospheric vertical motions. This implies that geo-engineering schemes aimed at reducing global warming without removing carbon dioxide from the atmosphere would fail to fully mitigate precipitation changes in the tropics. Strategies that may help constrain rainfall projections are suggested

Land–Ocean Shifts in Tropical Precipitation Linked to Surface Temperature and Humidity Change

“…In the tropics, fast adjustments to CO 2 forcing are expected to produce reductions in precipitation over ocean and increases over land associated with heat transport from land to ocean that attempts to maintain a near-time-invariant ratio of land to ocean surface temperature change (Lambert et al 2011;Bony et al 2013). These effects are significant in GCMs, linked to land surface warming and therefore at least partially represented by our method, but are small compared with SST-driven changes (Chadwick et al 2014).…”

“…In an effort to understand the differences, researchers have employed a number of schemes or ''decompositions'' that express precipitation changes in terms of a number of physically interpretable components (e.g., Emori and Brown 2005;Bony et al 2013;Chadwick et al 2013Chadwick et al , 2014Wills et al 2016). Decompositions have been derived in a variety of ways, but most have components identified as ''thermodynamic,'' which are due to changes in atmospheric moisture, and ''dynamic,'' which are due to changes in the intensity and location of atmospheric circulation features.…”

Climate tipping points and solar geoengineering

“…High ocean carbon concentrations result in ocean acidification, which can lead to damages (Brander et al (2012)). High atmospheric carbon concentrations may yield benefits (Pongratz et al (2012)) or damages (Bony et al (2013)). Just like with damages from SGE, these damages are mostly unknown.…”

Robust Hadley Circulation changes and increasing global dryness due to CO 2 warming from CMIP5 model projections

“…Reduction in midtropospheric relative humidity and clouds in the subtropics and midlatitude under global warming have also been noted in models and observations, suggesting the importance of cloud feedback and circulation changes (14-16). Even though robust global warming signals have been found in changing rainfall characteristics (2,17,18), in the widening of the subtropics, and in the relative contributions of circulation and surface warming to tropical rainfall from climate model projections and observations (19)(20)(21)(22)(23)(24), the dynamical linkages between HC changes and global patterns of moistening and drying have yet to be identified and understood. In this paper, we aim at establishing a baseline understanding of the dynamics of changes in the HC and relationships with increased global dryness based on monthly outputs from 33 Coupled Model Intercomparison Project Phase 5 (CMIP5) 140-y projection experiments under a scenario of a prescribed 1% per year CO 2 emission increase.…”