Some atmospheric gases, such as water vapour and CO2, absorb and re-emit infrared energy from the atmosphere down to the Earth’s surface. This process, the greenhouse effect, leads to a mean surface temperature that is 33 °C greater than it would be in its absence. If it were not for the greenhouse gas effect, Earth’s average temperature would be a chilly -18 °C. However, it is the non-condensable or long-lived greenhouse gases – mainly CO2, but also methane (CH4), nitrous oxide (N2O) and halocarbons (CFCs, HCFCs, HFCs) – that act as the drivers of the greenhouse effect. Water vapour and clouds act as fast feedbacks – that is to say that water vapour responds rapidly to changes in temperature, through evaporation, condensation and precipitation.
This strong water vapour feedback means that for a scenario considering a doubling of the CO2 concentration from pre-industrial conditions, water vapour and clouds globally lead to an increase in thermal energy that is about three times that of the long-lived greenhouse gases. Therefore, measured in the ability to trap the heat emanating from the Earth’s surface, water vapour and clouds are the largest contributors to warming. The amount of water vapour in the atmosphere is a direct response to the amount of CO2 and the other long-lived greenhouse gases, increasing as they do.
It is impossible for us to control directly the amount of water vapour in the atmosphere since water is found everywhere on our planet – it covers 71% of Earth’s surface. To limit the amount of water vapour in the atmosphere and control Earth’s temperature, we must limit the greenhouse gases that we can, in practice, do something about: CO2 and the other long-lived greenhouse gases.
GAW observes water vapour as it is an important atmospheric constituent through the role it plays in the climate system as a potent greenhouse gas and as a source for clouds. Water vapour is also important as a chemical compound, both in the troposphere as a source of the hydroxyl radical, the most important oxidant in the troposphere, and in the stratosphere where it has an influence on ozone depletion, especially in the Polar Regions.
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Balloon-borne sounding payload consisting of a NOAA frost point hygrometer (FPH, front), an electrochemical concentration cell (ECC) ozonesonde (rear) and an InterMet radiosonde (left). The thin stainless steel air inlet tube extends out the top of the FPH. A similar tube is fixed to the bottom of the FPH before launch. |
Measurement of water vapour
Atmospheric water vapour can be measured using a wide range of techniques and observational platforms. These observations are primarily used for numerical weather prediction, monitoring and research into climate and atmospheric chemistry. Water vapour is measured in situ by balloon- and aircraft-based instruments, and remotely by satellite- and ground-based sensors.
The different techniques for measuring water vapour include the use of:
- Passive microwave sensors installed on polar orbiting platforms;
- Infrared sensors, which constitute the longest satellite record of water vapour profiling and sounding instruments;
- Ultraviolet/visible/near-infrared imagers (daytime retrieval methods that use two channels and provide high spatial resolution (~ 1 km));
- Limb sounding, the technique of sounding various layers of the atmosphere by observation along a tangent ray that does not intersect the Earth’s surface;
- Radiosondes, a commonly used instrument for in situ sounding that provides high-quality profiles of relative humidity (among other variables) at a still unmatched vertical resolution of approximately 5 metres – on a global scale about 1 000 radiosondes are launched each day. The humidity sensors on radiosondes give good-quality humidity data throughout most of the troposphere, however, important corrections must be applied to their humidity measurements in the upper troposphere and stratosphere;
- Balloon-borne frost point hygrometers that use a cooled mirror, whose temperature is carefully controlled at the frost point temperature;
- Ground-based instruments, which allow for semi-continuous probing of the air mass over a fixed location; and
- Various long-range commercial aircraft equipped with vapour sensors.
Trends in observed atmospheric water vapour are hampered by inhomogeneities in data records, which occur when measurement programmes are discontinued because of, for example, the limited lifespans of satellite missions or insufficiently documented or understood changes in instrumentation. Combining records from different instruments that do not agree with one another is also a problem. One example is the offset between records from the HALOE and MLS satellite instruments. Nevertheless, observations show a steady increase of the total water vapour column as well as a 30-year net increase in stratospheric water vapour.
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Emrys Hall (CIRES, University of Colorado) prepares to launch a balloon carrying a NOAA FPH, an ECC ozonesonde and a radiosonde from the Marshall Field Site in Boulder, Colorado. Water vapour in climate models
During the latter half of the twentieth century the amount of water vapour in the stratosphere showed a net increasing trend, but since 2000 there have been periods of both increasing and decreasing abundance (Nedoluha et al., 2013). A comprehensive understanding of all the mechanisms driving changes in stratospheric water vapour is currently lacking. Most of the transport of gases from the troposphere to the stratosphere happens through the tropical tropopause. Due to the low temperatures in this region of the atmosphere, the air gets freeze-dried and very little water enters the stratosphere. In fact, an important source of stratospheric water vapour is the oxidation of methane transported up from the troposphere. Future warming due to climate change and increasing concentrations of methane are both expected to lead to more water vapour in the stratosphere.
Increases in water vapour in the upper troposphere and lower stratosphere (UTLS) lead to radiative cooling at these levels and induce warming at the surface. Recent analyses suggest that warming at the Earth’s surface may be sensitive to sub- parts per million (ppm) by volume changes in water vapour in the lower stratosphere. Research has found that a 10% decrease in stratospheric water vapour between 2000 and 2009 acted to slow the rate of increase in global surface temperature over this time period by about 25% compared to that which would have occurred due only to CO2 and other greenhouse gases.1 More limited data suggest that stratospheric water vapour probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapour is an important driver of decadal global surface climate change.
In the absence of global three-dimensional observations of water vapour, global reanalysis products are often used to validate numerical model simulations. Two extensively used reanalysis data sets are the NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA), its newest release MERRA2, and European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalyses.
A recent study showed that reanalysis data on high-altitude atmospheric water vapour, critical for the greenhouse effect, are not as accurate as previously thought. Water vapour data for the UTLS region from these reanalysis data sets have been compared to water vapour data from the Microwave Limb Sounder (MLS) on the AURA satellite. These satellite data have not been used in the production of these reanalysis, so they represent an independent data set well suited for validation. The study found that the reanalyses differed quite a lot from the MLS observations, overestimating the annual global mean water vapour in the upper troposphere by about 150%. Vertically, water vapour transport across the tropical tropopause (16–20 km) in the reanalyses is faster by up to ~86% compared to MLS observations. In the tropical lower stratosphere (21–25 km), the mean vertical transport from ECMWF is 168% faster than the MLS estimate, while MERRA and MERRA2 have vertical transport velocities within 10% of MLS values. Horizontally at 100 hectopascal (hPa), both MLS observations and reanalyses show faster poleward transport in the Northern Hemisphere than in the Southern Hemisphere. Compared to MLS observations, the water vapour horizontal transport for both MERRA and MERRA2 is 106% faster in the Northern Hemisphere but about 42–45% slower in the Southern Hemisphere. ECMWF horizontal transport is 16% faster than MLS observations in both hemispheres.
To add complexity to these discrepancies it should also be mentioned that water vapour data from MLS show dry biases of 10-20% in the tropical upper troposphere compared to frost point hygrometers launched on weather balloons from Hilo, Hawaii, and San José, Costa Rica (Dale Hurst, 2016). The MLS dry biases may slightly reduce the wet biases in the MERRA and ERA Interim reanalyses with respect to MLS.
 Stratospheric water vapour trends over Boulder, Colorado, show a 30-year net increase in stratospheric water vapour. From Hurst et al., 2011 |
These large discrepancies between different types of observational data, and between observations and reanalysis results, demonstrate significant uncertainties in the measurements as well as our lack of understanding of the transport and dehydration processes in the UTLS region. They also show that there is a great need for more and better observations of water vapour in this region. As mentioned in the section on measurements, the current observational systems are hampered by various shortcomings, such as the limited lifetimes of satellite missions and a sparse spatiotemporal distribution of balloon- and ground-based measurements; for example, there is only one site in the world (Boulder, Colorado) where there is a 30+ year time series of balloon measurements of water vapour in the UTLS region.
The models that are used to predict future climate use reanalysis data to verify that the current climate is modelled correctly. The lack of accurate water vapour data in the important UTLS region will therefore limit the ability of these models to predict future climate.
Water vapour as a chemical compound
In addition to acting as a greenhouse gas and as a source of clouds, water molecules also take part in chemical reactions in the atmosphere. Water vapour, together with ozone, is an important source for the formation of the highly reactive hydroxyl radical (OH). The OH radical is the most important oxidant in the lower atmosphere, providing the dominant sink for many greenhouse gases (e.g., CH4, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs)) and pollutants (e.g., CO and non-methane hydrocarbons). In clean air, the OH radical is formed through this pair of chemical reactions:
O3 + n (I<340nm) –> O2 + O(1D)
O(1D) + H2O –> 2OH
The abundance of OH in the atmosphere depends on the amounts of ozone and water vapour. OH production also depends on the amount of overhead ozone as this determines the amount of short-wave radiation needed to crack the ozone molecule.
Whereas the troposphere is quite moist, the stratosphere is very dry, typically with water vapour mixing ratios ≤ 5 ppm. This means that usually there are no clouds in the stratosphere. However, if temperatures drop below -78°C a special type of water and nitric acid (HNO3 • 3H2O) ice clouds can form. On the surfaces of the ice particles, chemical reactions occur that convert innocuous chlorine reservoir compounds (hydrochloric acid, HCl and chlorine nitrate, ClONO2) to reactive forms (chlorine monoxide, ClO) that destroy ozone.
Increasing concentrations of water vapour together with decreasing temperatures in the stratosphere – also a consequence of climate change – will give rise to more of these clouds and that will lead to more severe ozone depletion as long as the concentration of ozone depleting gases remains high.
 Mother-of-pearl clouds in the stratosphere, about 20-25 km above the ground, form in lee-waves when strong westerly winds blow over the Norwegian mountains. The colours are caused by diffraction around the ice particles that make up these clouds. Despite their beauty they forebode ozone destruction through the conversion of passive halogen compounds into active species that destroy ozone. |
Challenges in observing water vapour
The distribution of water vapour in the upper troposphere and the stratosphere is not very well known due to a paucity of observations in this region of the atmosphere. The global distribution of water vapour in the upper troposphere and the stratosphere is not very well known due to a paucity of high vertical resolution observations in this region of the atmosphere. In some cases there are also significant discrepancies between satellite data, frost point hygrometer data and meteorological reanalyses. More accurate data with better geographical coverage is needed. The observed temporal trends in stratospheric water vapour are poorly understood and this demonstrates our lack of understanding of how water vapour enters the stratosphere. These are areas that GAW will address in the future.
References
Forster, P. M. de F., and K. P. Shine (2002), Assessing the climate impact of trends in stratospheric water vapor, Geophys. Res. Lett., 29, 1086, doi:10.1029/2001GL013909.
Hurst, D.F., S.J. Oltmans, H. Vömel, K.H. Rosenlof, S.M. Davis, E.A. Ray, E.G. Hall and A.F. Jordan, 2011: Stratospheric water vapor trends over Boulder, Colorado: Analysis of the 30 year Boulder record. Journal of Geophysical Research: Atmospheres, 116(D2):D02306, doi:10.1029/2010JD015065.
Hurst, D.F., 2016, personal communication.
Jiang, Jonathan H., Hui Su, Chengxing Zhai, Longtao Wu, Kenneth Minschwaner, Andrea M. Molod, Adrian M. Tompkins, 2015: An assessment of upper troposphere and lower stratosphere water vapor in MERRA, MERRA2, and ECMWF reanalyses using Aura MLS observations, J. Geophys. Res. Atmos., 120, 11,468–11,485, doi:10.1002/2015JD023752.
Lacis, A.A., J.E. Hansen, G.L. Russell, V. Oinas and J. Jonas, 2013: The role of long-lived greenhouse gases as principal LW control knob that governs the global surface temperature for past and future climate change. Tellus B, 65:19734, doi10.3402/tellusb. v65i0.19734.
Nedoluha, G. E., Michael Gomez, R., Allen, D. R., Lambert, A., Boone, C., and Stiller, G.: Variations in middle atmospheric water vapor from 2004 to 2013, J. Geophys. Res. Atmos., 118, 11285–11293, doi:10.1002/jgrd.50834, 2013.
Solomon, S., K. H. Rosenlof, R. Portmann, J. Daniel, S. Davis, T. Sanford, and G. -K. Plattner (2010), Contributions of stratospheric water vapor to decadal changes in the rate of global warming, Science, 327, 1219-1223, doi:10.1126/science.1182488.
Further reading
Observations of water vapour: N. Kämpfer (ed.), Monitoring Atmospheric Water Vapour, ISSI Scientific Report Series 10, DOI 10.1007/978-1-4614-3909-7, ©Springer Science+Business Media, LLC 2013
Water vapour as a greenhouse gas and as a feedback: https://www.skepticalscience.com/water-vapor-greenhouse-gas.htm
Authors
Ed Dlugokencky, U.S. National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory
Sander Houweling, Netherlands Institute for Space Research (SRON)
Ruud Dirksen, Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) Lead Centre, Deutscher Wetterdienst (DWD)
Marc Schröder, Deutscher Wetterdienst (DWD)
Dale Hurst, U.S. National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado
Piers Forster, School of Earth and Environment, University of Leeds
WMO Secretariat, Oksana Tarasova, Chief, and Geir Braathen, Senior Scientific Officer, Global Atmosphere Watch
Footnotes
1 Solomon, S., K. H. Rosenlof, R. Portmann, J. Daniel, S. Davis, T. Sanford, and G.-K. Plattner (2010), Contributions of stratospheric water vapour to decadal changes in the rate of global warming, Science, 327, 1219–1223, doi:10.1126/science.1182488.
