Michael Byrne

Graduate student | MIT Program in Atmospheres, Oceans and Climate

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Land-ocean warming contrast

Fig 1: Modeled changes in surface temperature and relative humidity under global warming (CMIP5, RCP8.5)

Observations and simulations of climate change show a pronounced land-ocean contrast in surface warming. This is not just a transient, heat capacity effect - models that are run out to equilibrium maintain a warming contrast (though the climate models disagree on the magnitude of the contrast). The ratio of land to ocean warming can be up to 1.5 at certain latitudes with obvious impications for the regional and societal impacts of climate change.

We have developed a simple theory to understand the physical mechanisms controlling the magnitude of the warming contrast and its intermodel scatter in the tropics. The theory is based on both the weak temperature gradient approximation and convective quasi-equilibrium and can accurately estimate the warming contrast as a function of the base-state climate and the changes in land and ocean relative humidity (we have recently applied our theory to an idealized climate model and to CMIP5 models). Changes in land relative humidity, which are anti-correlated with temperature changes (Fig 1), contribute strongly to the magnitude of the contrast and account for much of the intermodel scatter. Challenges remain in understanding the physics controlling the warming contrast outside the tropics and in estimating how land relative humidity changes with climate. A conference presentation summarizing this research can be found here.

Water cycle in a changing climate

Fig 2: Modeled changes in precipitation minus evaporation (solid line) over land (red) and ocean (blue). The thermodynamic scalings (Held & Soden, 2006) are also plotted (dashed line)

It is widely predicted that radiative forcing resulting from rising levels of greenhouse gases in the Earth's atmosphere will lead to important changes in the hydrological cycle and surface temperature distribution. Observations and model simulations indicate that this climate response is substantially different over land and ocean regions.

Changes in the hydrological cycle over ocean are closely tied to temperature changes via a simple thermodynamic scaling; the so-called “rich-get-richer” mechanism (Fig 2). Over land, however, where there is limited moisture availability, the behavior is more complex, with the thermodynamic scaling failing to capture important features of the precipitation minus evaporation response (the rich shouldn't necessarily expect to get richer). We are using a hierarchy of tools, starting with pencil+paper and ending with complex climate models, to understand this behavior of the terrestrial water cycle.