Hiljattain julkaistu tutkimus (Ø. Hodnebrog et al.: Water vapour changes due to different climate drivers) jossa on kaiken kaikkiaan 16 eri instituuttia eri puolilta maailmaa, jotka avaavat käsityksiä vesihöyryn vaikutuksesta ilmastoon, missä ensimmäisten joukossa:
CICERO (Center for International Climate Research, Oslo, Norway / PhD. Gunnar Myhre)
Ohessa suora lainaus abstraktista:
Water vapor in the atmosphere is the source of a major climate feedback mechanism and potential increases in the availability of water vapor could have important consequences for mean and extreme precipitation. Future precipitation changes further depend on how the hydrological cycle responds to different drivers of climate change, such as greenhouse gases and aerosols.
Currently, neither the total anthropogenic influence on the hydrological cycle nor that from individual drivers is constrained sufficiently to make solid projections.
We investigate how integrated water vapor (IWV) responds to different drivers of climate change. Results from 11 global climate models have been used, based on simulations where CO2, methane, solar irradiance, black carbon (BC), and sulfate have been perturbed separately.
While the global-mean IWV is usually assumed to increase by ∼ 7% per kelvin of surface temperature change, we find that the feedback response of IWV differs somewhat between drivers. Fast responses, which include the initial radiative effect and rapid adjustments to an external forcing, amplify these differences.
The resulting net changes in IWV range from 6.4 ± 0.9% K−1 for sulfate to 9.8 ± 2% K−1 for BC. We further calculate the relationship between global changes in IWV and precipitation, which can be characterized by quantifying changes in atmospheric water vapor lifetime.
Global climate models simulate a substantial increase in the lifetime, from 8.2 ± 0.5 to 9.9 ± 0.7 d between 1986–2005 and 2081–2100 under a high-emission scenario, and we discuss to what extent the water vapor lifetime provides additional information compared to analysis of IWV and precipitation separately.
We conclude that water vapor lifetime changes are an important indicator of changes in precipitation patterns and that BC is particularly efficient in prolonging the mean time, and therefore likely the distance, between evaporation and precipitation.”
- Tutkimus siis määrittelee, miten vesihöyry käyttäytyy erilaisten ajurien vaikutuksesta. Tutkimuksessa on hyödynnetty 11:sta eri ilmasto-mallia, joka perustuu CO2:n, metaanin, auringon irradianssin, mustan hiilen ja sulfaatin vaikutukseen.
- Toinen oleellinen tekijä on vesihöyryn eliniän muutokset ilmakehässä
WVLi = IWVi / Pi
ΔWVL = WVLi – WVL base
WVLS = ΔWVL / ΔTs
WVLi = vesihöyryn elinikä (päivissä)
IWVi = vesihöyryn globaali integroitu keskiarvo (kg m-2)
Pi = vesihöyryn globaali saostumisen keskiarvo (kg m -2 d-1), häiriötekijällä i
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Results and discussion
3.1 Zonal- and annual-mean changes in IWV
Table 1 shows that the slow responses of global-mean water vapor per kelvin of change in surface temperature are fairly close to the 7% K−1 that we expect from the Clausius– Clapeyron relation. However, the numbers differ somewhat between drivers, ranging from 6.5 ± 1% K−1 for SO4x5 to 8.1 ± 1% K−1 for Sol+2%.
These differences become larger when the fast response is included, which adds to the pure surface-temperature-related response. The fast response is largest for BCx10, which changes from 7.5 ± 1% K−1 to 9.8 ± 2% K−1 between the slow and total response.
Integrated water vapor increases much more than evaporation and precipitation at nearly all latitudes and for all five PDRMIP drivers (Figs. 1, S2–S3). However, the total global mean increase in IWV differs strongly between each driver, with BCx10 at 9.8±2%K−1 and SO4x5 at 6.4±0.9%K−1 (Table 1). The estimated global IWV increase for BCx10 ranges from 6.8 to 13% K−1 for the different PDRMIP models, while locally decreasing in some regions (Fig.S4). BCx10, and to some extent SO4x5, show steep north–south gradients in the IWV change, emphasizing the strong regional influences of these short-lived compounds (Figs. 1, S3–S4). The north–south gradient for BCx10 is steeper for the total response (Fig. 1b) than the slow response (Fig. 1a) due to strong influence of the fast response for this com- pound. In contrast to the other climate drivers, precipitation decreases and water vapor increases strongly for BCx10.
Based on new model simulation data we have investigated how different climate drivers influence water vapor in the atmosphere. We find that the feedback response of IWV, the relative change per kelvin of global- and annual-mean surface temperature change, differs somewhat between drivers, ranging from 6.5%K−1 for sulfate to 8.1%K−1 for solar forcing. Fast responses are particularly important for black carbon because of rapid heating of the atmospheric column, leading to an increase from 7.5% K−1 to 9.8% K−1 between the feedback response and the total IWV response, and with strong regional differences in the IWV distribution. For CO2, fast responses are also important, leading to a decrease from 7.7% K−1 to 7.2% K−1, for the slow and total response respectively, partly due to a reduction of relative humidity throughout the troposphere in the fast response. We also show that the fast response is an important contributor to the previously known strong land–ocean contrast in the response of near-surface relative humidity to global warming, with CO2 and BC showing strong and opposite fast responses over land.
Results show that the lifetime of water vapor could increase by 25% by the end of the 21st century in a high-emission scenario. This is because of the large expected temperature changes, and despite the projected aerosol emission reductions leading to a lower water vapor lifetime sensitivity. Among the climate drivers studied here (CO2, methane, solar irradiance, BC, and sulfate), WVL changes are most sensitive to perturbations in BC aerosols (1.1±0.4dK−1 increase in Ts), due to strong increases in IWV with temperature combined with a precipitation reduction (in contrast to a positive precipitation change per unit temperature change for other drivers). According to model calculations, an increase in WVL of 4 %–5% between pre-industrial and present day has already occurred, and around half of this increase is due to fast atmospheric responses. Aerosol concentration changes, and BC in particular, strongly modify the fast WVL change and contribute to large inter-model uncertainty.
The increase in WVL with global warming reveals important changes in the hydrological cycle.
Quantifying WVL changes gives information about changing precipitation patterns – information that cannot be deduced by analyzing IWV and P separately. More specifically, a longer lifetime leads to greater distances between the source (evaporation) and sink (precipitation) of water vapor, with implications such as the Hadley cell expansion (Singh et al., 2016). Our results show that BC is considerably more efficient than any of the other climate drivers in prolonging the WVL, and therefore likely the transport length of water vapor. Estimating WVL could become more important in the future as inclusion of isotopes in GCMs becomes more common, and this may lead to more robust projections of the hydrological cycle and precipitation.
Kuten tutkimuksesta voidaan todeta, Clausius-Clapeyron differentiaaliyhtälö, -jonka avulla voidaan arvioida kyllästyneen vesihöyryn höyrynpaineen eli vesihöyryn kriittisen osa-paineen käyttäytymisen edistymistä eri lämpötila-alueilla, soveltuu erinomaisesti approksimaatioihin (≈ 7 – 8% K-1).
(ks. artikkelikuvan graafit)