Water Cycle Varies over Land and Sea

Abstract

The salinity at the ocean surface has changed over the past 50 years; saline regions have become more saline, whereas regions of relatively fresh water have become even fresher. In their Report “Ocean salinities reveal strong global water cycle intensification during 1950 to 2000” (27 April, p. [455][1]), P. J. Durack et al. conclude that these changes indicate a change in the difference between evaporation and precipitation. In his related News & Analysis story “The greenhouse is making the water-poor even poorer” (27 April, p. [405][2]), R. A. Kerr applies this result to the land surface, based on the idea that the global water cycle is dominated by the oceanic component. Kerr asserts that wet land got wetter and dry land got drier with more floods and droughts. The measurement of ocean salinity and link to the oceanic component of the water cycle is a scientific advance. However, its interpretation requires proper context: One does not normally think of droughts and floods over the oceans. The assumption that the land must be behaving in the same way as the ocean is almost certainly wrong. In the global water cycle, evaporation generally exceeds precipitation over the ocean, balanced by precipitation exceeding evaporation over land. Over land, the excess water is the river runoff that closes the cycle. In the oceans, evaporation can exceed precipitation because water is always available for evaporation. In contrast, the land is not always wet and thus evaporation cannot steadily exceed precipitation in the absence of irrigation or groundwater use. Over dry land, the difference between precipitation and evaporation is near zero—there is little runoff. Defining “wetness” as precipitation minus evaporation ([ 1 ][3]) means that over land, dry places tend to remain dry while wet places could become either wetter or drier ([ 2 ][4]). In a literal sense, the ocean surface is always wet, whereas much of the land surface is dry and therefore operates differently. In short, we need to stop the simple extrapolation of results from ocean studies to the land and vice versa. 1. [↵][5]1. I. M. Held, 2. B. J. Soden , J. Clim. 19, 5686 (2006). [OpenUrl][6][CrossRef][7][Web of Science][8] 2. [↵][9]1. M. L. Roderick, 2. G. D. Farquhar , Water Resour. Res. 47, W00G07 (2011). [OpenUrl][10][CrossRef][11] [1]: /lookup/doi/10.1126/science.1212222 [2]: pending:yes [3]: #ref-1 [4]: #ref-2 [5]: #xref-ref-1-1 View reference 1 in text [6]: {openurl}?query=rft.jtitle%253DJ.%2BClim.%26rft.volume%253D19%26rft.spage%253D5686%26rft_id%253Dinfo%253Adoi%252F10.1175%252FJCLI3990.1%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [7]: /lookup/external-ref?access_num=10.1175/JCLI3990.1&link_type=DOI [8]: /lookup/external-ref?access_num=000242163800014&link_type=ISI [9]: #xref-ref-2-1 View reference 2 in text [10]: {openurl}?query=rft.jtitle%253DWater%2BResour.%2BRes.%26rft.volume%253D47%26rft.spage%253DW00G07%26rft_id%253Dinfo%253Adoi%252F10.1029%252F2010WR009826%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [11]: /lookup/external-ref?access_num=10.1029/2010WR009826&link_type=DOI