One of the repeating nightmares about global warming is that the current very pokey rate of sea level rise will suddenly accelerate. Now, it turns out that multiple lines of evidence say this has not happened and isn’t likely to, either.
Recently, Science magazine reported that glacial flow in Greenland has not been accelerating as fast as previously reported (Moon et al., 2012). The major implication is that the contribution of ice loss from Greenland to global sea level rise is not increasing at the rate once expected. Now, Geophysical Research Letters (GRL) reports that glacier loss in the Russian high Arctic is contributing about 0.025 mm of sea level rise per year, but that contribution has likely been largely unchanged for at least 30 years (Moholdt et al., 2012). More from GRL (Levitus et al., 2012) is that the rate of increase in the ocean’s heat content—which raises sea level—has recently slowed. And finally, from a soon-to-be-published paper in GRL comes word that the net non-climate contributions of human activity to sea level rise have been speeding up (Wada et al., 2012).
Here, I’ll tie them all together and tell you what they mean.
I’ll focus on the Wada et al. (2012) paper because from the results presented there, we can derive the global implications of all the others.
Wada and colleagues, in a paper titled “Past and future contribution of global groundwater depletion to sea-level rise,” examine how much human removal of water from deep aquifers (for irrigation, etc.), also known as “dewatering” of the continents—water that eventually finds it way into the sea—has contributed to observed sea level rise from 1900 through 2000, as well as how much it may contribute in the future (through 2100).
I have discussed previous work from Yoshihide Wada (https://www.cato.org/the-current-wisdom‑2/) and there has been a complimentary analysis done by Leonard Konikow of the U.S. Geological Survey (see here for details, http://www.masterresource.org/2011/09/rapid-sea-level-rise-nature-no/ ).
The take-home message from those articles was that the “dewatering” was adding a significant, growing, but often overlooked, input of water to the global oceans and was responsible for a non-negligible amount of sea level rise.
In fact, between the Wada and Konikow calculations, the contribution was estimated to range from 15 percent to 25 percent of the current rate of sea level rise, which stands at about 2.5 mm/year (a rate which has been declining in recent decades, see here http://sealevel.colorado.edu/).
You would think that a factor contributing such a substantial proportion to current sea level rise wouldn’t be overlooked, after all, the media hyperventilated last week when a report came out about the potential speedup of glaciers in Antarctica which currently contribute ~0.25 mm/year of sea level rise—a value about half the current groundwater depletion contribution.
In what is becoming a depressingly repetitive pattern, “big science” assessments of climate change, like those made by our government, or those of the United Nations, simply ignore legitimate findings that don’t fit with the established (end of the world) meme.
According to Wada et al. (2012) “[i]n the IPCC fourth assessment report, the contribution of non-frozen terrestrial waters to sea-level variation is not included due to its perceived uncertainty and assumption that negative contributions such as dam impoundment compensate for positive contributions (mainly from groundwater depletion).” This situation is drastically changing. Wada et al. continue “However, recent work on global groundwater depletion [Wada et al., 2010; Konikow, 2011] suggests a rapid increase of this positive contribution to sea-level rise during the last decade that warrants a re-appraisal of the contribution of terrestrial water and in particular groundwater depletion to projected 21st century sea-level change.”
As indicated in the quote above, human activity contributes to changes in sea level in two ways besides any impact from climate change. The first is through the pumping of water from aquifers at a rate greater than is the replenishment rate, and the second is through water impoundment—that is, building dams to hold water than normally would have been in the ocean. The former acts to increase sea level, the latter acts to decrease it. But, the contribution from impoundment is a one-off deal for each dam because once it is built and the reservoir filled, the water then flows through as before. The contribution, however, from deep aquifer pumping is on-going.
Wada and colleagues derive a long-term record, extending back to 1900, of both impoundment and dewatering on a global level, and make projections as to the future course of these two processes across the 21st century (Figure 1). The dark blue line which peaks in the 1960s and 1970s is the annual contribution to sea level rise from water impoundments (in Figure 1, the sign is flipped for this process), the light blue area is the annual contribution (including estimate uncertainty) to global sea level from dewatering. The grey line in Figure 1 is the combination of two (plus some other minor elements) which represent the net change in global sea level annually supplied by human activity not associated with significant climate change.
Figure 1. History of the estimated and projected annual contribution of terrestrial water storage change to global sea-level over the period 1900–2100 (figure from Wada et al., 2012).
Notice in Figure 1 that the grey line (the net contribution) rises above zero in the early 1980s—meaning that since then, human activity has been putting more water in the ocean than we are holding back. Also notice that our net positive contribution to sea level rise has been rapidly rising since then.
Consider the above in light of the one of the favorite climate change alarmist talking points—that the rate of sea level rise is accelerating and instead of perhaps a foot of sea level rise by century’s end, we should be expecting 3 feet or more.
Let’s look at the data. Figure 2 shows the latest-greatest sea level rise history as assembled by John Church and colleagues (Church and White, 2011; red line) along with the same thing once we remove net human contribution from impoundment and dewatering (blue line). Notice that the shapes of the two curves are a bit different after about 1950 (when the direct human contribution starts to be significant—see Figure 1). The red curve (total sea level) appears to be slightly cupped upwards (that is, accelerating), while the blue curve (sea level less direct human contribution) appears more linear (i.e., constant).
Figure 2. Observed change in sea level, 1900–2010. Raw sea level values (red); sea level after removing contribution from impoundments and continental dewatering (blue) (data from Church and White, 2011; Wada et al., 2012).
Figure 3 shows the running 10-yr trend through each of the two datasets, beginning with data in 1950. The red curve (which is the raw sea level data) shows an upward trend (again, indicating an acceleration in the rate of sea level rise), with the highest values at the end of the curve. On the other hand, the blue curve (which is the raw sea level less the direct human contribution), shows no such upwards trend (indicating that no acceleration) and the current rate of sea level rise (right-hand end of the curve) is neither the highest, nor far from being unique.
Figure 3. 10-yr moving linear trend through the raw sea level values (red), and the sea level after removing the contribution from impoundments and continental dewatering (blue), 1960–2009 (data from Church and White, 2011; Wada et al., 2012).
What this means is that the apparent acceleration in the rate of sea level rise has been caused solely by the changes in the direct contribution from human activity (which continues to increase) and not by climate change.
This makes perfect sense given the other papers I listed at the beginning of the article, which indicate modest increases from the world’s glacial fields along with a modest decrease in the rate of thermal expansion as the build-up of heat content in the oceans slows, reflecting the hiatus in global temperature rise that began over 15 years ago.
So much for another alarmist talking point. This one simply doesn’t hold water.
References:
Church, J. A. and N.J. White, 2011. Sea-level rise from the late 19th to the early 21st Century. Surveys in Geophysics, doi:10.1007/s10712-011‑9119‑1. Data available, http://www.cmar.csiro.au/sealevel/sl_data_cmar.html
Konikow, L., 2011. Contribution of Global Groundwater Depletion Since 1900 to Sea-Level Rise. Geophysical Research Letters, 38, L17401, doi:10.1029/2011GL048604.
Levitus, S., et al., 2012.World ocean heat content and thermosteric sea level change (0–2000), 1955–2010, Geophysical Research Letters,doi:10.1029/2012GL051106, in press.
Moholdt, G., B. Wouters, and A. S. S. Gardner, 2012. Recent mass changes of glaciers in the Russian High Arctic, Geophysical Research Letters, doi:10.1029/2012GL051466, in press.
Moon, T., I. Joughin, B. Smith, and I. Howat, 2012. 21st-century evolution of Greenland outlet glacier velocities. Science, 336, 576–578, doi:10.1126/science.1219985
Prichard, H.D., et al., 2012. Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature, 484, 502–505, doi:10.1038/nature10968.
Wada, Y., et al. 2010. Global Depletion of Groundwater Resources. Geophysical Research Letters, 37, L20402, doi:10.1029/2010GL044571.
Wada, Y., et al., 2012. Past and future contribution of global groundwater depletion to sea-level rise. Geophysical Research Letters, doi:10.1029/2012GL051230, in press.
