Peter Molnar
Ph.D. Columbia University, 1970
Professor, Geological Sciences
E-mail: molnar@cires.colorado.edu
Office: ESCI 462C
Phone: 303-492-4936
Web: Molnar Research Group
Research Interests
Large-scale deformation of continents, crustal and mantle geodynamics, climate change (especially over geological time scales), mechanisms of erosion, interactions among geodynamics, climate change, and geomorphology
Current Research: Tibet and the Asian Monsoon System
Figure 2: (top) Upper-tropospheric temperature (250
hPa) over Asia in July; the maximum overlies northern
India and Pakistan, not the Tibetan Plateau. (bottom)
The moist entropy in July on a terrain-following model
level within 50 hPa (about 500 m) of the surface. All
quantities are means for 1979–2002 from the ERA-40
reanalysis dataset.
I devoted a part of my research effort in FY10 to synthesizing evidence that pertains to the growth of the Tibetan Plateau and to the Asian monsoon, in modern and geologic time. (Molnar et al. 2010).
The extreme breadth and height of the Tibetan Plateau are commonly assigned key roles in the Asian monsoon system, and hence most imagine that the geologic history of the Asian monsoon is closely related to the growth of Tibet. According to this view, heating of the air immediately above Tibet induces ascent and cross-equatorial circulation that comprises the upper branch of the monsoon circulation. A new view is emerging, however, in which the role of Tibet is little more than a barrier to flow of cool dry air from northern Eurasia, and heating over the plateau plays a minor role, at least in the South Asian (or Indian) monsoon. Together with William Boos of Harvard University and David Battisti, I reviewed not only this new view, but also the geologic history of Tibet and the Asian monsoon (Molnar et al. 2010).
One of the breakthroughs in geology in the past 20 years has been the development of methods for determining paleoelevations. When applied to Tibet, virtually all studies show elevations comparable to present-day elevations, with the one exception in northern Tibet (Figure 1). If a part of Tibet rose recently, since about 10 million years ago, when some evidence suggesting a strengthening of the monsoon occurred, that part must be northern Tibet. The view that heating of Tibet plays a key role in the strength of the monsoon loses some credibility when upper atmospheric temperatures are plotted (Figure 2); the hottest upper troposphere is not over Tibet, but to its south over northern India. Moreover, current theories hold that the edge of the monsoon circulation should lie over the region of highest specific entropy, which also lies not over Tibet, but over northern India (Figure 2). It seems that the Himalaya, the southern edge of Tibet, plays a key role by blocking cool dry air from farther north, air that would reduce the specific entropy of air over India if it could interact with the hot moist air formed over the Indian subcontinent. That blockage, not the heating of Tibet, allows the South Asian monsoon to become very strong. If so, the growth of Tibet is unlikely to have played a key role in the development of the South Asian monsoon.
This review presents a summary of the recent thinking on both the growth of Tibet and its significance for the geologic history of the Asian monsoon.
Professor Molnar is a CIRES Professor.
