Interviews with leading experts on Climate, Glaciers, and Greenland ice.
From the IPCC Third Assessment Report (TAR) in 2001:
The Antarctic ice sheet is likely to gain mass because of greater precipitation, while the Greenland ice sheet is likely7 to lose mass because the increase in runoff will exceed the precipitation increase.
Concerns have been expressed about the stability of the West Antarctic ice sheet because it is grounded below sea level. However, loss of grounded ice leading to substantial sea level rise from this source is now widely agreed to be very unlikely during the 21st century, although its dynamics are still inadequately understood, especially for projections on longer time-scales.
In the intervening years, more data has became available, especially gravity measurements from the GRACE satellite system. But in 2007, the 4th assessment Report (AR4) still qualified sea level rise estimates with the caveat “excluding rapid dynamical changes in ice flow” – an important point that climate deniers have deliberately chosen to ignore and exploit.
See more on this in Richard Alley’s recent lecture, which I’ll be revisiting in a future vid. Dr Alley is more concerned about West Antarctica than Greenland for the coming century. However, since 2012’s record surface melt on Greenland, and the emerging GRACE data showing Greenland with a 10 year doubling time on mass loss, there has been a renewed conversation about Greenland dynamics, and possible contributions to Sea level this century.
In a recent update, Dr. James Hansen warned specifically about change in Greenland mass loss that could lead to more rapid contributions than has been generally discussed.
Perceived authority2 in the case of ice sheets stems from ice sheet models used to simulate paleoclimate sea level change. However, paleoclimate ice sheet changes were initiated by weak climate forcings changing slowly over thousands of years, not by a forcing as large or rapid as human-made forcing this century. Moreover, in a paper submitted for publication (Hansen et al., 2013) we present evidence that even paleoclimate data do not support the degree of lethargy and hysteresis that exists in such ice sheet models.
Fig. 1 (above) shows the Greenland ice sheet mass change estimated by Shepherd et al. (2012). The input-output method calculates the difference between mass gained through snowfall and mass lost by sublimation, meltwater runoff, and discharge of ice into the ocean. This record and the analysis of satellite gravity measurements agree within their margins of error (see Shepherd et al., 2012). There are no satellite gravity data to confirm or refute the large amplitude of fluctuations prior to 2000 in Fig. 1, which are based on input-output calculations.
Fig. 1 shows that Greenland has been losing mass at a faster and faster rate over the past decade, with the recent rate corresponding to ~1 mm sea level per year (1 mm sea level = 360 Gt ice). The linear fit to the Shepherd et al. data in Fig. 1 yields a Greenland contribution to global sea level of about 30 cm by 2100.
The increasing Greenland mass loss in Fig. 1 can be fit just as well by exponentially increasing annual mass loss, a behavior that Hansen (2005, 2007) argues could occur because of multiple amplifying feedbacks as an ice sheet begins to disintegrate. A 10-year doubling time would lead to 1 meter sea level rise by 2067 and 5 meters by 2090. The dates are 2045 and 2057 for 5-year doubling time and 2055 and 2071 for a 7-year doubling time.
In light of Hansen’s calculations, the observations noted in the video, of an observed 10 year doubling by Dr. Jason Box are sobering. Dr Eric Rignot sounds a note of caution due to the magnitude of the 2012 melt, which may be an outlier. Nevertheless, Dr. Mike MacCracken, Dr. Box, and Dr. Tom Wagner have all alluded to an ongoing re-evaluation of of outlet glacier melt versus surface melt in Greenland dynamics.
For a short summary of the Sea Level kerfuffle, see my ClimateCrocks sea level vid from 2009.
This coming September, a new IPCC report will be released. A successful release will be accompanied by an intelligent social media rollout campaign to head off such obvious points of exploitation for the denialist media. More on that in coming months as well.
Kobashi (2011, http://www.leif.org/EOS/2011GL049444.pdf): “Excluding the last millennium, there were 72 decades warmer than the present one, in which mean temperatures were 1.0 to 1.5°C warmer, especially in the earlier part of the past 4000 years [Dahl‐Jensen et al., 1998; Wanner et al., 2008]. During two intervals (∼1300 B.P. and ∼3360 B.P.) centennial average temperatures were nearly 1.0°C warmer (−28.9°C, the 97 percentile) than the present decade (Figure 1, bottom). From the above observations, we conclude that the current decadal mean snow temperature in central Greenland has not exceeded the envelope of natural variability of the past 4000 years.“
I also recommend this graph:
http://3.bp.blogspot.com/_WWYveRtoZBA/S2Q4DQw7naI/AAAAAAAAAbY/JL6OJpL5cl0/s1600/greenland-2KBC-2008.png (Greenland temperature of 2.5 thousand. BC to 2008).
This graph shows us that the temperature rise in the Northern Hemipshpere from 1800 to 2008 was not at all unnatural. That rise follows precisely the long-term pattern, where such rises have been occurring approximately every 1,500 +/- 500 years.
http://bprc.osu.edu/geo/publications/mckay_etal_CJES_08.pdf :
“More importantly, there have been times when sea-ice cover was less extensive than at the end of the 20th century.
It is important to note that the amplitude of these millennial-scale changes in sea-surface conditions far exceed those observed at the end of the 20th century.”
http://blog.chron.com/climateabyss/category/weather-climate/texas-drought-weather-climate/ :
“Knowing that the odds of a certain type of event have increased anthropogenically is an abstract sort of knowledge. […]”
One paper, based on one line of evidence in one very small region. And a few blog posts. Given the body of proxy work on Holocene temperatures, the documented and very rapid temperature rise over the last 150 years, the basic physical relation between CO2 and heat retention as well as ocean acidity, our heavy reliance on climate stability for food production and the oceans for food, and our apparent aim of burning every bit of fossil fuel we can get our hands on, this kind of idle dismissal is (to put it very gently) irresponsible.