Asian Water Towers: More on Monsoons—Response

Abstract

Pithan raises two valid concerns. We agree that future precipitation is highly uncertain and that general circulation models (GCMs) have difficulty in capturing monsoon precipitation ([ 1 ][1], [ 2 ][2]), and we stress that in our Report. For all five of the river basins we analyzed, the multimodel average (MMA) of future precipitation shows limited changes both in total quantity and temporal shifts. The Yellow river basin is an exception, and the MMA shows a positive offset in total winter precipitation. However, there is considerable variation between the different GCMs. Therefore, we do not conclude that a reduction in melt water is offset by an increase in precipitation, but by an increase in rainfall. Given the projected increase in temperature, which is less uncertain than precipitation projections, more precipitation will fall in the form of rain. Because the melt season coincides with the rain season in most regions, this compensates the reduced melt. Pithan disagrees with our use of a constant degree-day factor. His argument pertains to the spatial variation in melt rates within a basin. We agree in principle that this would be an important factor to consider in projections, and we did not consider it explicitly, due to lack of information about local albedo trends. However, we did implicitly account for uncertainty about the degree-day factor by using an uncertainty analysis around snow and ice degree-day factors (described in the supporting online material of our Report). The case for which melt rates would be higher because of lower albedo would be in the higher regions of our uncertainty envelopes. Moreover, we point out that our extreme case (without any glaciers) results in similar conclusions as our best-guess case. 1. [↵][3]1. H. Annamalai, 2. K. Hamilton, 3. K. R. Sperber , J. Clim. 20, 1071 (2007). [OpenUrl][4][CrossRef][5] 2. [↵][6]1. S. Yang 2. et al ., J. Clim. 21, 3755 (2008). [OpenUrl][7][CrossRef][8] [1]: #ref-1 [2]: #ref-2 [3]: #xref-ref-1-1 View reference 1 in text [4]: {openurl}?query=rft.jtitle%253DJ.%2BClim.%26rft.volume%253D20%26rft.spage%253D1071%26rft_id%253Dinfo%253Adoi%252F10.1175%252FJCLI4035.1%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 [5]: /lookup/external-ref?access_num=10.1175/JCLI4035.1&link_type=DOI [6]: #xref-ref-2-1 View reference 2 in text [7]: {openurl}?query=rft.jtitle%253DJ.%2BClim.%26rft.volume%253D21%26rft.spage%253D3755%26rft_id%253Dinfo%253Adoi%252F10.1175%252F2008JCLI1961.1%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 [8]: /lookup/external-ref?access_num=10.1175/2008JCLI1961.1&link_type=DOI