Water in the Atmospherebjorn Stevens and Sandrine Bony

Water in the atmosphereBjorn Stevens and Sandrine Bony

Much of what we know, and even more of what we don’t know, about Earth’s climate and its propensity to change is rooted in the interplay between water, air circulation, and temperature.

very schoolchild learns about the role of how you look at it, being suspended in the atmos- the atmosphere in Earth’s water cycle. But phere is an exceedingly unlikely state for a water few get the chance to learn about water’s molecule to find itself in; but while in that state, E role in determining the properties of the water makes a world of difference. atmosphere. Water determines not only how the Sun’s energy is partitioned through the at- An absorption virtuoso mosphere and across Earth’s surface but also the Water stands out because of its physical and radia- character of the large- scale circulations, which that tive properties. As figure 1 shows, it is a small mol- energy drives. Recognition of the fundamental in- ecule with a large appetite for IR radiation. The fluence of water even predates formal scientific water molecule is endowed with a plethora of rota- thinking. In the Judeo-Christian creation myth, for tional absorption modes, which result from the tum- instance, one of the creator’s first tasks, after sepa- bling of its strong electric dipole around three small rating darkness from light, was to separate water and disparate moments of inertia. These modes con- from water to create the sky. tribute to a rich set of spectral lines that stretch from the near-IR into the microwave. Some of them arise That water assumes such a defining place in the because the rotational modes ornament three vibra- sky is remarkable given that it only accounts for tional modes that form the fundamental rotational– 0.25% of the total mass of the atmosphere. That’s the vibrational (ro-vibrational) bands. One, λ = 6.3 μm, equivalent of a liquid layer only 2.5 cm deep, barely 2 is associated with H-O-H bending; the other two, λ enough to make a global puddle, distributed 1 and λ3, associated with symmetric and asymmetric through the atmosphere almost entirely (99.5%) in stretching, are located near 2.7 μm and overlap with the form of vapor. By way of comparison, the global an overtone of the bending mode. ocean, if spread uniformly over Earth’s surface, would have an average depth of about 2.8 km. Fresh Bjorn Stevens directs the Max Planck Institute for Meteorology in Hamburg, water on Earth’s terrestrial surface—ice sheets, lakes, Germany, and Sandrine Bony is a CNRS senior scientist at the Laboratoire de rivers, wetlands, and soils—is 2000 to 3000 times Météorologie Dynamique in Paris. more abundant than atmospheric water. No matter RKCRSESN PORKERI CHRISTENSEN, ERIK

www.physicstoday.org June 2013 Physics Today 29

Downloaded 31 May 2013 to 136.172.75.107. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://www.physicstoday.org/about_us/terms Water

Such a surfeit of absorption lines also endows strong greenhouse effect and a strong planetary the water molecule with a potent continuum absorp- albedo, despite containing only about 0.5% of the at- tion. Debate continues about the relative contribu- mosphere’s water.2 As only a small fraction of that tions to that continuum from the overlap of the water is distributed in large, precipitating hydro - wings of different spectral lines and from the tran- meteors such as rain and snow, which have relatively sient production of molecular clusters of water small surface-to-volume ratios, precipitating water dimers during collisions in the vapor (see PHYSICS is far less important for Earth’s radiative budget than TODAY, April 2013, page 18). Whatever the cause, the water vapor or suspended condensate. continuum absorption is an important manifestation of water vapor’s unusual effectiveness at interacting Thermodynamics and phase changes with radiation throughout the thermal IR.1 This ef- Because water cycles through vapor and condensate fectiveness is felt also in the near-IR and makes water phases in the atmosphere, the laws of thermodynam- vapor the most important absorber of solar radiation ics place important constraints on the coupling be- in the lower atmosphere. Other molecules found in tween water vapor and air temperature. Through the atmosphere also have strong, or well-placed, the Clapeyron equation, the second law of thermo- absorption features in the IR. But when it comes to dynamics dictates how the saturation vapor pressure interacting with the full spectrum of IR radiation, es depends on temperature T and fundamentally con- including overtones at shorter wavelengths, none strains the humidity structure of the atmosphere. The approaches the virtuosity of the water molecule. equation can be expressed as (See the article by Raymond Pierrehumbert, PHYSICS TODAY, January 2011, page 33.) β delns = dT ln . Condensed water, which atmospheric physi- T (1) cists refer to collectively as hydrometeors, can take various forms, such as the simple crystals, droplets, The factor β is roughly equal to the ratio of the en- snowflakes, graupel, and hail illustrated in figure 1. thalpy of vaporization—the energy required to The extent to which they scatter electromagnetic ra- transform water from liquid to gas at constant pres- diation depends on their refractive index and their sure—to the water vapor gas constant. Its value size relative to the radiation’s wavelength: Shorter (about 5400 K) expresses the strength of the effect of waves are preferentially scattered, longer waves are temperature variations on saturation vapor pres- absorbed. Because the typical size of cloud droplets sure. Because of water’s unusually large enthalpy of (and to a lesser extent cloud ice) is commensurate vaporization, β is a factor of two or three larger than with the shorter wavelengths of the thermal IR, that of other common condensable vapors such as clouds are effective at absorbing energy at these and carbon dioxide, methane, and ammonia. longer wavelengths. The large vaporization enthalpy also implies

They are much less effective in doing so at the that at typical surface temperatures, es approximately 10- to 100-fold shorter wavelengths found in the doubles for every 10-K rise in temperature. If the

solar part of the spectrum, where the radiation is in- vapor pressure e rises above es—for instance, as a stead scattered. As a result, clouds exhibit both a result of expansional cooling or a radiant loss of

a Suspension Precipitation

Crystals Snowflakes

Graupel and hail Water molecule Figure 1. Hydrometeors (a), the condensed forms of water in the Drizzle atmosphere, come in several Cloud droplets Raindrops sizes. They mostly scatter visible light but absorb over a broad −3 −2 −1 0 1 2 3 4 10 10 10 10 10 10 10 10 range of the IR. (b) The near- b DIAMETER (μm) and thermal-IR regions of the spectrum excite the molecule Visible Near-IR Thermal IR and produce its rotational– vibrational (or ro-vibrational) Ro-vibrational Rotational and rotational bands. Specific 100 λ λ λ lines 1, 2, and 3 mark the Ro-vibrational symmetric stretching mode, TY overtones 1 bending mode, and asymmetric VE LINE stretching mode, respectively.

INTENSI

RELATI 0.01

λ λ λ 0.4 1.0 3 1 4.0 2 10.0 40.0 WAVELENGTH λ (μm)

30 June 2013 Physics Today www.physicstoday.org

ambient vapor pressure near (Pa) its saturation value. For short- e 102 lived and dilute condensate clouds, the condensate largely follows the atmospheric flow until it evaporates again as the 101 temperature rises or the vapor pressure falls—for instance,

through mixing with drier air. VAPOR PRESSURE, 0 For clouds that are suffi- 10 ciently long- lived or condensate laden, larger hydromete- 200 220 240 260 280 300 ors such as rain or snow form TEMPERATURE (K) and rapidly precipitate from Figure 2. The atmospheric vapor pressure (a) the atmosphere. as a function of temperature is bounded by The first law of thermo- the atmosphere’s vapor pressure when saturated with water (solid line). The data are shown dynamics dictates how tem- as the median (dot) and range (error bar) of 228 monthly values at different isobaric levels— perature changes for adia- 900 hPa (green), 700 hPa (blue), 500 hPa (orange), and 300 hPa (red). (Data are provided by batic displacements of air the European Centre for Medium-Range Weather Forecasts.) (b) Based on satellite measure- parcels and thus fundamen- ments of radiation emitted at 6.2 μm, this map reveals the relative humidity of the middle tally constrains the thermal and upper troposphere. It also reveals how closely humidity levels are tied to atmospheric structure of the atmosphere. circulation patterns: Pronounced dry regions (dark) show large-scale descending motions of Because air has mass, its pres- air concentrated in the subtropics. Clouds, regions of complete saturation, are visible as sure decreases with altitude. white features. (Image from Meteosat.) So as a parcel of air ascends, it expands and cools adiabatically; likewise, descend- vapor phase, making saturated air quite dry. So ing air warms. The adiabatic temperature change that relatively dry regions of the atmosphere are in- accompanies such vertical displacements is deter- dicative of descending currents, which adiabati- mined by γ*, the adiabatic temperature lapse rate. cally warm and thereby lower their relative hu- * −1 For effectively dry air, γ = γd = g/cp≈10Kkm , midity (e/es). Such descending currents are where g is the gravitational acceleration and cp is the preferentially located in the subtropics, which ex- isobaric specific heat capacity of the air. The adiabatic plains why the warm regions at a given isobaric lapse rate of moist air, γm, is less than γd. The differ- level tend to be relatively dry, as figure 2b bears ence, which in today’s atmosphere can be more than out. Hence, temperature places an upper but not a factor of two, arises because condensation, and a lower limit on the atmospheric humidity, and the hence latent heating, accompanies adiabatic cooling degree of saturation is strongly connected to at- as an air parcel expands. (See the Quick Study by Dale mospheric circulation. Durran and Dargan Frierson, PHYSICS TODAY, April 2013, page 74.) The rate of condensation is linked to Understanding climate through water the rate of temperature change by equation 1, making Water’s radiative properties determine the magni- it straightforward to derive that tude of the greenhouse effect, the planetary albedo, and hence Earth’s surface temperature. As we’ll see, e β s those properties also determine the strength of the ∂T 1 + p (T ( γγ≡≈−,(2) hydrological cycle and influence the thermodynamic md∂z e R β 2 structure of the troposphere, the lower 10–15 km of 1 + pcs d p (T ( the atmosphere. Taken together, the effects from the presence of water also provide a foundation for understanding broad characteristics of the atmospheric where Rd is the specific gas constant in dry air and p is the ambient pressure. At warm temperatures circulation, particularly in the tropics. −1 A moist atmosphere is largely transparent to near Earth’s surface, γm ≈4K km , but it approaches visible light but opaque to the IR. The enthalpy—or γd in the cold and desiccated regions higher in the atmosphere. heat content, roughly—that’s radiated to space by The strong coupling between water and tem- atmospheric water must thus be balanced by an perature is evident in figure 2a, which shows that input of enthalpy from Earth’s surface. But radiative the ambient water vapor pressure e is bounded— transfer alone does not maintain that balance. In an over four orders of magnitude—by its saturation atmosphere that contains sufficient water, radiative

value es. One can think of the circulation of air and transfer can only maintain the balance if the actual its humidity as also coupled. Whether a parcel of air lapse rate γ is large—much larger than γm. In that is relatively moist or dry depends on its history—in situation, the adiabatic displacement of saturated particular, the temperature when it was last satu- air would make the air warmer than its surround- rated with water. In the cold of high altitudes, for ings and hence unstable. Vigorous convective cur- instance, very little water can be sustained in the rents, either in the form of towering cumulus clouds

www.physicstoday.org June 2013 Physics Today 31

Downloaded 31 May 2013 to 136.172.75.107. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://www.physicstoday.org/about_us/terms Water

γ , at some altitude water molecules be- a Themodynamic structure b Enthalpy budget m come so scarce that their net radiative contribution is negligible. That altitude Top of atmosphere Solar Terrestrial defines the upper limit of the radiatively irradiance irradiance driven convection layer and the height 200 K of the troposphere. The temperature at the top of the troposphere is thus rela- ~ 15 000 m tively independent of the temperature Saturated updrafts at the surface, and the tropospheric height adjusts to maintain consistency

with the lapse rate γm. The difference between γd and γm imposes an asymmetry on vertical dis-

ALTITUDE placements in the atmosphere. In air that becomes just saturated, upward 74% 48% displacements will be accompanied by 26% an adiabatic temperature change that 300 K

700 m follows γm, while that from downward HUMIDITY TEMPERATURE displacements will follow γd. Because Figure 3. The thermodynamic structure and enthalpy budget of the atmosphere. the thermal structure of the descend- (a) The atmosphere’s temperature (red) and its absolute humidity (blue) are closely ing zones of the atmosphere is tied to coupled. (b) At the top of the atmosphere solar and terrestrial irradiances balance one that of the ascending zones—that is, another. According to calculations, most (74%) of the incoming solar irradiance reaches γ ≈ γm—the difference between the ac- the surface, but the net terrestrial irradiance at the surface is only a small fraction (26%) tual lapse rate and the adiabatic lapse of its value at the top of the atmosphere. The radiative deficit (48%) is balanced by rates in those regions must adjust to surface turbulent fluxes of enthalpy, arising mostly from evaporation, that transport balance the radiative cooling. That bal- warm water vapor from the surface to the troposphere, where it cools and condenses. ance dictates the rate of descent, w, to satisfy Q ≈ w(γd − γm), where Q is the radiative cooling rate. That equation in the tropics or eddies in the mid latitudes, rapidly helps atmospheric physicists understand why a respond to the slow destabilization by radiative relatively small fraction of the atmosphere con- processes and mix together layers of the tropo- tains ascending currents. sphere until the air reaches a convectively neutral The power of the constraints imposed by

state, at which γ ≈ γm. water is borne out by idealized, but nonetheless The convective currents transport water up- quantitative, studies of RCE. Despite a great many ward to much lower pressure and temperature, simplifications, these studies reproduce the main where it condenses and precipitates from the atmos- features of Earth’s atmosphere,5 including its ther- phere. Over time, the net condensational heating modynamic structure and enthalpy budget. They must balance the radiative cooling (see figure 3). It also confirm that water is at least as important an is in this sense that one can understand that the influence on the atmosphere’s structure and circu- strength of the hydrological cycle is determined by lation patterns as are Earth’s rotation, the presence the rate at which the atmosphere radiatively cools. of continents, and poleward gradients in solar in- And as has been appreciated for more than a cen- solation. According to computer simulations, tury, the troposphere is best described as being in a making water invisible to radiation leads to a very state of radiative convective equilibrium (RCE) un-Earth-like atmosphere—one for which the sur- rather than radiative equilibrium.3 Less appreciated face would be cooler, on average, by more than is the role water plays, through its interaction with 20 K and in which convection plays almost no role IR radiation, in demanding such a balance. in the transfer of enthalpy. The atmosphere would Thermodynamic constraints accompanying find itself in a state of near radiative equilibrium the presence of water help dictate basic features and thus relative stasis. Put simply, if water wasn’t of the atmospheric thermal structure near RCE. radiatively active, there would be no need for rain. Upward-directed convective currents produced by Calculations like those that generated the en- radiative cooling rapidly saturate with water and thalpy-budget numbers in figure 3, or more exact

adopt a temperature profile roughly equal to γm. approaches based on cloud irradiances measured Because γm < γd, the downward branches of the cir- by satellites, can be used to distinguish the contri- culation must stably equilibrate to the thermal butions of water vapor to the energy budget from structure of the upward branches rather than vice those of water condensate (in the form of clouds). versa. That equilibration is carried out by gravity Both approaches demonstrate that in a globally waves, which act to eliminate density differences averaged sense, clouds act to cool Earth’s surface. (essentially due to temperature) in air along iso- That result arises nontrivially out of the balance baric surfaces. The process is particularly efficient of two large but opposing effects: the cloud- in the tropical troposphere.4 greenhouse effect, whose magnitude can be meas- Because water vapor pressure decreases as tem- ured by the difference between the cloud-top temperature decreases, as shown in figure 2, and temperature and the surface temperature; and the perature decreases with altitude, roughly following cloud-albedo effect, whose magnitude depends on

32 June 2013 Physics Today www.physicstoday.org

Downloaded 31 May 2013 to 136.172.75.107. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://www.physicstoday.org/about_us/terms the amount of incident sunlight and differences feedback, surface warming prompts the planetary between the reflectivities of the cloud and Earth’s albedo to decrease as snow and sea ice melt. surface. The water-vapor and lapse-rate feedbacks During the day, most clouds cool the surface, have opposing signs and are linked, though the particularly low-level clouds whose greenhouse ef- water-vapor feedback is considerably larger in fect is small. At night, all clouds warm the surface. magnitude. Indeed, even after subtracting the The differences between the irradiances from a fully lapse-rate feedback, the water-vapor feedback, clouded sky and a clear sky can be used to quantify which accompanies an atmosphere that warms the clouds’ opposing effects. At the top of the atmos- while maintaining a constant relative humidity, phere, satellites measure the cooling albedo effect to is still as large as, or larger than, the cloud- 2 be about 50 W/m , about twice as large as the warm- greenhouse and surface-albedo feedbacks put to- ing greenhouse effect. On balance, clouds moderate gether, as figure 4 shows. The strength of the the substantial warming of the surface that arises water- vapor feedback rests on the observation that 6,7 from the presence of water vapor. the relative humidity change little with tempera- The net radiative effects of clouds also have ture; that is, the vapor pressure changes closely the potential to influence the strength of the hy- follow the change in saturation vapor pressure, as drological cycle. Although in today’s climate, the figure 2 shows.3,10,11 Hence, the presence of water net radiative heating of the atmosphere attributa- in the atmosphere amplifies any external radiative ble to clouds is nearly zero, when averaged glob- forcing; that fact has been at the centerpiece of ally. Clouds can have a large effect on the radiative our understanding of climate change for more heating rate regionally. And in so doing they help than 30 years.12 shape the structure and pattern of large-scale cir- Amplification of a radiative forcing isn’t the culation systems.8 Whether or not that circulation only consequence of water’s presence. The hydro- helps ensure that the clouds’ globally averaged radiative effect on atmospheric heating is near zero logical cycle can also change in response to those ex- remains a mystery. ternal forcings, and our understanding of water helps explain those changes. Because the radiative Amplifier of climate change cooling of the troposphere is an increasing function Because water is so closely coupled to temperature, of its temperature, a warmer atmosphere cools more it cannot drive climate change. Rather, it orches- rapidly. And because at the top of the atmosphere trates and amplifies the effects of agents capable of the IR flux emitted to space must balance the ab- acting independently of surface temperature. For sorbed solar radiation flux, additional atmospheric instance, as concentrations of long- lived green- cooling occurs through a reduction of the net upward thermal irradiance at the surface. To the extent house gases such as CO2 increase, so does the IR opacity of the atmosphere, which in turn raises the that the increased cooling cannot be offset by in- altitude z* from which thermal radiation is emitted creased absorption of solar radiation by water to space. Such a change—referred to as radiative vapor, or by a change in cloudiness, it must be as- forcing—produces an imbalance in Earth’s radia- sociated with an increase in global precipitation. tion budget. The balance can be restored either by Regionally, wind currents will mediate where increasing the temperature at z* or by increasing the that increase occurs. If the currents change little planetary albedo. In the former case, the constraints compared to the amount of water they transport, on the lapse rate require that the surface tempera- wet regions will import more atmospheric moisture ture must also increase. and become wetter; conversely, dry regions will ex- Simple calculations show that if only tempera- port more moisture and become, on average, ture was allowed to change in response to a dou- drier.10,13 Those circulation effects are governed by

bling of atmospheric CO2, surface temperature equation 1 and combine with the energetic con- would increase by a little more than 1 K. The pres- straints to provide a good first approximation to ence of water amplifies the response in several well- changes in precipitation, understood ways (see figure 4). In the so-called water- vapor feedback, an increase in temperature β δP δR P E δ T supports an increase in the water vapor pressure ≈ + T ( − ) lnsfc , (3) throughout the troposphere, which further in- sfc creases the IR opacity of the atmosphere. That, in turn, further increases z*, which raises temperatures where δR is the change in the net cooling rate of the further, and so on. atmosphere, P and E measure precipitation and The lapse rate γ decreases in magnitude with evaporation (in enthalpy flux units), respectively, warming because of the temperature dependence of and Tsfc is the surface temperature. Equation 3 helps explain the precipitation changes predicted by γm in equation 2. In the lapse- rate feedback, that decrease reduces the surface temperature change complex models and gives confidence in the assess- implied by a given change in z*. Through positive ment that regional changes will accompany global cloud-greenhouse feedback, surface warming pro- warming.14 The last term of equation 3 emphasizes duces a greater temperature difference between the how closely precipitation is coupled to changes in surface and the top of the troposphere. It thereby in- atmospheric circulation patterns, which are them- creases the greenhouse potential of clouds to further selves coupled to water in ways that remain poorly warm the surface.9 Finally, in the surface-albedo understood.

www.physicstoday.org June 2013 Physics Today 33

Downloaded 31 May 2013 to 136.172.75.107. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://www.physicstoday.org/about_us/terms Water

Robust feedbacks Figure 4. How much does the temperature of Earth’s surface change from doubling the carbon dioxide in the atmosphere? Comprehensive 1.15 2.67 climate models from the coupled-model intercomparison project T Δ d a b c (CMIP5) yield a median estimate of 3.45 K bounded in the orange box T Δ water by the 25%–75% spread over the total range of values. Shown in blue are the estimates of the temperature change based on well-understood feedback processes. In the absence of the effects of water, one expects a T 3.45 warming Δ d of about 1 K. Water in the atmosphere increases that dry CMIP5 modeling response by an additional 1.5 K. The amplification includes contributions from processes described in the text: the combined water- vapor and lapse- rate feedbacks (a), the cloud- greenhouse feedback (b), and the 0 1 2 3 4 surface- albedo feedback (c). SURFACE TEMPERATURE CHANGE( K)

Cloudy futures scales as short as a few hours.18 More generally, so strong is the coupling between clouds and circula- Scientists have good reasons to believe that changes tion systems, from thunderstorms to monsoons, to water in response to a given radiative forcing ac- that advancing our understanding of regional cli- count for more than half of Earth’s average surface mate change rests firmly on advancing our under- temperature change. But estimating the climate sen- standing of clouds and cloud processes. sitivity with greater precision remains difficult. Despite imperfect models, our understanding of Models that encapsulate the basic properties of water the behavior of the climate system is so deeply rooted produce a wide range of estimates (see figure 4). Most in the basic physicochemical properties of the water of the imprecision in climate sensitivity and re- molecule that we can confidently conclude that global gional patterns of rainfall changes can be related to warming from anthropogenic emissions of long-lived a poor understanding of how clouds change in a greenhouse gases poses serious risks. And yet we’re warming climate15 and how changing clouds affect 8 hampered by an inability to clearly pin down the pace atmospheric circulations. of that warming and the nature of regional changes the Although clouds have long been recognized as planet is likely to experience. A grasp of both is crucial crucial for Earth’s radiation budget, only in the past for adaptation measures. That fact highlights the ur- few decades have researchers appreciated that gent need to better understand the ways in which clouds can both warm and cool the atmosphere and water couples to the atmosphere’s circulation systems. the surface. Early models of the climate system, to the extent that they considered clouds at all, assumed We thank Kerry Emanuel, Isaac Held, John Mitchell, and that their effect on the enthalpy budget was not a Raymond Pierrehumbert for their extensive and thoughtful function of the climate state. But RCE models devel- comments on an early version of this manuscript, and Aiko oped in the early 1970s and later general-circulation Voigt for contributions to our discussion of RCE. models demonstrated that clouds exert a marked influence on climate sensitivity.8,16 References In addition to the idea of a positive feedback as- 1. K. P. Shine, I. V. Ptashnik, G. Rädel, Surv. Geophys. 33, sociated with changes in the cloud-greenhouse effect, 535 (2012). 2. R. T. Pierrehumbert, Principles of Planetary Climate, other ideas have begun to emerge as to why cloudi- Cambridge U. Press, New York (2010). ness might depend on the working temperature of 3. S. Manabe, R. T. Wetherald, J. Atmos. Sci. 24, 241 (1967). the atmosphere. In most cases the ideas stem from the 4. K. A. Emanuel, J. D. Neelin, C. S. Bretherton, Q. J. R. fundamental properties of water. For instance, be- Meteorol. Soc. 120, 1111 (1994). cause the lapse rate of air that remains saturated as it 5. D. Popke, B. Stevens, A. Voigt, J. Adv. Model. Earth Syst. (in press), doi:10.1002/jame.20009. rises is a function of temperature, warmer climates 33 might be characterized by more condensate-laden 6. B. Stevens, S. E. Schwartz, Surv. Geophys. , 779 (2012). 7. G. L. Stephens et al., Nat. Geosci. 5, 691 (2012). clouds; the larger optical depth increases the cloud- 8. A. Slingo, J. M. Slingo, Q. J. R. Meteorol. Soc. 114, 1027 albedo effect and thereby moderates the warming. In (1988). contrast, warming is also expected to be accompa- 9. D. L. Hartmann, K. Larson, Geophys. Res. Lett. 29, 1951 nied by increased evaporation, which drives more (2002). mixing in the lower atmosphere and may lead to 10. J. F. B. Mitchell, C. A. Wilson, W. M. Cunnington, 113 fewer clouds, enhancing warming.17 Q. J. R. Meteorol. Soc. , 293 (1987). 11. S. C. Sherwood et al., J. Geophys. Res. 115, D09104 (2010). As the singular challenge clouds pose to our 12. S. Bony et al., in Climate Science for Serving Society: understanding of climate and climate change has Research, Modelling, and Prediction Priorities, G. Asrar, become better appreciated, research on clouds has J. W. Hurrell, eds., Springer, Berlin (in press). intensified. In recent years detailed experimentation 13. I. M. Held, B. J. Soden, J. Climate 19, 5686 (2006). and analyses of climate models have demonstrated 14. S. Bony et al., Nat. Geosci. (in press), doi:10.1038/ngeo1799. 19 which cloud regimes and processes are critical to ex- 15. S. Bony et al., J. Climate , 3445 (2006). 16. V. Ramanathan, J. Coakley Jr, Rev. Geophys. Space Phys. plaining intermodel differences in the projections of 16 15 , 465 (1978). future climate. Recent research also shows that 17. M. Rieck, L. Nuijens, B. Stevens, J. Atmos. Sci. 69, 2538 clouds directly mediate the response of the atmos- (2012). phere to an external forcing, and they do so on time 18. J. Gregory, M. Webb, J. Climate 21, 58 (2008). ■

34 June 2013 Physics Today www.physicstoday.org

Downloaded 31 May 2013 to 136.172.75.107. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://www.physicstoday.org/about_us/terms