Since glacier mass loss is the largest single cause of sea level rise, this explains why there has been no acceleration of sea level rise over the past 100-200 years and why sea level rise is also unlikely to accelerate in the 21st century.
The lack of any acceleration in glacier mass loss and sea level rise in the 20th century despite an exponential rise in greenhouse gases proves that both of these processes are primarily natural and unrelated to CO2. During prior interglacials, most glaciers disappeared, as well as the entire ice sheet of Greenland and West Antarctica, all entirely natural and with "safe" levels of CO2. There is no evidence the current interglacial is any different.
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Glaciers have lost a substantial fraction of their mass during
the past century (Cogley, 2009; Marzeion et al., 2012), with
the globally averaged mass balance turning negative probably
around 1850 (Leclercq et al., 2011).Within the 20th century,
mass loss of glaciers was likely the largest single cause
of sea level rise, followed by thermal expansion of the ocean,
mass loss of the Greenland and Antarctic ice sheets, and
changes in terrestrial water storage (Gregory et al., 2013).
Even though the rise of global mean air temperature accelerated
in the 20th century, the mass loss rate of glaciers during
the second half of the 20th century was not higher than
during the first half of the century (Leclercq et al., 2011;
Marzeion et al., 2012).
Observed sea level rise during the 20th century can only be
explained if the glaciers’ contribution did not develop in parallel
to global mean temperature, but was high already in the
first half of the 20th century (Gregory et al., 2013). Our result
that changes in glacier hypsometry play a significant role in
shaping the glaciers’ response to climate change, in particular
that loss of low-lying surface area (i.e., terminus retreat to
higher elevations) decreases the sensitivity is critical for explaining
the strong glacier-mass losses during the first half of
the 20th century.
Conclusions
We have used a model of glacier response to climate change
to quantify the equilibrium sensitivity of glaciers, and to distinguish
the respective contributions of temperature and precipitation
anomalies. Because of the geographic distribution
of glaciers, the temperature and precipitation change experienced
by glaciers is far greater than the global mean. Precipitation
anomalies projected for the future dampen the mass
loss of glaciers, but their effect is strongly limited by the increasing
temperatures, which increases the liquid fraction of
precipitation on the glaciers.
We find that glacier-mass loss during the 21st century is
to a significant degree a response to 20th century climate
change. This partly explains the relatively weak dependence
of 21st century mass loss on future greenhouse gas emissions.
A second reason is that the complete loss of individual
glaciers imposes a strong restriction to the rates of mass loss
in a warming climate. Results from methods not accounting
for finiteness of ice mass available for melting, for example,
by extrapolating current rates of mass loss, or even increases
of rates of mass loss (Meier et al., 2007), will therefore yield
substantial overestimates.
Thirdly, changes of glacier hypsometry reduce the response
of glaciers to warming, and need to be considered
in explaining the observed rates of mass loss during the 20th
century and the projected rates for the 21st century. Figure 9
summarizes our quantification of this effect. We find that the
retreat of glacier termini to higher altitudes is a strong negative
feedback, which is becoming weaker as more glaciers
disappear completely.
The Cryosphere, 8, 59-71, 2014 www.the-cryosphere.net/8/59/2014/ doi:10.5194/tc-8-59-2014 |
Feedbacks and mechanisms affecting the global sensitivity of glaciers to climate change
1Center of Climate and Cryopshere, Institute of Meteorology and Geophysics, University of Innsbruck, Austria
2Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland
3NCAS-Climate, University of Reading, Reading, and Met Office Hadley Centre, Exeter, UK
Abstract. Mass loss by glaciers has been an important contributor to sea level rise in the past, and is projected to contribute a substantial fraction of total sea level rise during the 21st century. Here, we use a model of the world's glaciers to quantify equilibrium sensitivities of global glacier mass to climate change, and to investigate the role of changes in glacier hypsometry for long-term mass changes. We find that 21st century glacier-mass loss is largely governed by the glacier's response to 20th century climate change. This limits the influence of 21st century climate change on glacier-mass loss, and explains why there are relatively small differences in glacier-mass loss under greatly different scenarios of climate change. The projected future changes in both temperature and precipitation experienced by glaciers are amplified relative to the global average. The projected increase in precipitation partly compensates for the mass loss caused by warming, but this compensation is negligible at higher temperature anomalies since an increasing fraction of precipitation at the glacier sites is liquid. Loss of low-lying glacier area, and more importantly, eventual complete disappearance of glaciers, strongly limit the projected sea level contribution from glaciers in coming centuries. The adjustment of glacier hypsometry to changes in the forcing strongly reduces the rates of global glacier-mass loss caused by changes in global mean temperature compared to rates of mass loss when hypsometric changes are neglected. This result is a second reason for the relatively weak dependence of glacier-mass loss on future climate scenario, and helps explain why glacier-mass loss in the first half of the 20th century was of the same order of magnitude as in the second half of the 20th century, even though the rate of warming was considerably smaller.
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