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Archive for the ‘sea level rise’ Category

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Natural climate factors unlikely to put the brakes on greenhouse-gas-driven sea level rise this century

Wednesday, March 10th, 2010

The IPCC 2007 report projected a conservative sea level rise of about 18-59 cm by the year 2100.

Why conservative?  Because it mainly accounted for things we know are happening and can measure well—like thermal expansion of the ocean and melting of land glaciers (see here for a discussion of the Kilimanjaro example).  What it doesn’t do so well is account for all of the potential ways that the big ice sheets (Greenland and Antarctica) can contribute to sea level rise.  Things like ice flow and mass loss are generally assumed to be constant, even though recent research papers discussed in previous posts (here and here) suggest they are accelerating.

Since the publication of the IPCC report in 2007, there have been several studies suggesting that sea level rise will be 1-2 meters or more by 2100 (one example here).  One study looked at geological evidence for sea level rise during the previous interglacial period 125,000 years ago, which was 1-2 degrees C warmer than today.  Their work indicated that there was a 95% chance that sea level rose by 6 meters (22 feet).

In a forthcoming issue of Geophysical Research Letters,  Svetlana Jevrejeva and colleagues used statistical models to project sea level rise by 2100.1 But they also did something else interesting.  They looked back several thousands of years to the most extreme events that could cause climate cooling—things like severe volcanic eruptions, which create stratospheric dust clouds that block sunlight.

If events like this were to happen again, they asked, would they cause enough cooling to be able to slow sea level rise caused by greenhouse gases?

The answer is no.  There appears to be no natural factors like vulcanism that will significantly slow greenhouse-gas-driven sea level rise that we are already committed to or future sea level rise that we may experience if we continue to emit fossil fuels.

Excerpts (emphasis mine):

  • With the assumption that sea level will continue to respond over the next 100 years to the same forcings that have influenced it during the past 1000 years, we estimate 0.6 -1.6 m of global sea level rise in the 21st century using a statistical model driven by projected natural and anthropogenic forcings.
  • In contrast to the 20th century sea level rise that was associated with a significant contribution of 25% from natural (solar and volcanic) forcing, 21st century sea level rise will be clearly dominated by the changes in CO2 and other greenhouse gases.
  • Alternative scenarios for solar forcing with a potential decrease in solar irradiance of 1W/m2 (using the lowest level recorded throughout the last 9300 years) only produce a 10-20 cm reduction in our estimate of 21st century sea level rise.
  • If we utilize the 13th century past volcanic forcing to estimate a possible (but unlikely) contribution from volcanic activity, then an almost negligible 8 cm decrease is projected in the estimated sea level rise.
  • The suggested reduction of radiative forcing by injections of SO2 into atmosphere (equivalent to a Pinatubo eruption every 4 years) would be equivalent to delaying sea level rise by 12 -20 years.
  • A “no changes in radiative forcing” scenario produced 16-22 cm (with lower limit of 10 cm and upper limit of 31 cm) sea level rise in the 21 century due to the inertia of the climate system, providing evidence that conditions established during the past centuries have already committed us to a considerable global sea level rise during the next 100 years.

1Jevrejeva, S., J. C. Moore, and A. Grinsted (2010). How will sea level respond to changes in natural and anthropogenic forcings by 2100? Geophysical Research Letters : 10.1029/2010GL042947

UPDATE:  RealClimate provides more explanation of the IPCC being too cautious about sea level rise.

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Posted in climate adaptation, climate change science, polar ice, sea level rise | No Comments »

What happened to sea level in the past when the planet warmed 1-2 degrees?

Sunday, December 20th, 2009

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A powerful tool that scientists use to determine impacts of climate warming is historical records from ice cores, ocean and lake sediments, fossil reef terraces, and tree rings.   These records have helped us determine that we are in a current warm (interglacial) phase—called the Holocene— of the Pleistocene Ice Age, a period spanning the last 2 million years, when Northern Hemisphere ice sheets advanced and retreated more than a dozen times.

In this week’s issue of Nature (subscription required), Robert Kopp and colleagues examined1 the previous warm interglacial phase, the Eemian, which happened about 125,000 years ago.  The historical records suggest that global temperature was about 1-2 degrees C warmer than today, and maybe as much as 3-6 degrees C warmer at the poles.

The question they asked was this:  How much did global sea level rise with the 1-2 degrees C  global warming in the Eemian?  Using the geological record to estimate past sea level changes, they came up with a startling answer:

  • 95% chance that sea level was 6.6 meters (22 feet) higher than today
  • 67% chance that it was 8 meters (26 feet) higher
  • 33% chance it was 9.4 meters (31 feet) higher

It’s important to remind ourselves that a 1-2 degree global warming is not some kind of scientific doomsday prediction—it’s actually the lower end of warming scenarios, pushing the limit of what is technologically and politically achievable.  If we continue a business as usual scenario, IPCC models suggest a global average warming of 4 degrees C by 2100.

1Kopp, R et al. (2009) Probabilistic assessment of sea level during the last interglacial stage. Nature 462: 863-868.

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Posted in climate change science, sea level rise | 1 Comment »

New sea level rise estimates—getting worse

Monday, December 7th, 2009

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Sea level is a notoriously difficult thing to predict.  Currently, it is not something that climate models (process models based on the transfer of heat and matter) are very good at because we are still learning how ice sheets and glaciers behave.

Therefore, scientists have taken a different approach, using simpler models that relate sea level height to temperature, which climate models are good at predicting.

In the early edition of this week’s Proceedings of the National Academy of Sciences (open access), Martin Vermeer and Stefan Ramstorf apply1 an updated version of one of these new models.

Several important points:

  • Thermal expansion of the ocean is a big driver of sea level rise—between 55-70%.
  • Even the low-emission scenario indicates sea level rising 1 m by 2100.
  • Higher emission scenarios suggest it may be as much as 2 m.
  • Both of these are about 3 times higher than the 2007 IPCC report suggests.
  • Like the IPCC report, this model does not include the possibility of more catastrophic thaw of major ice sheets like Greenland and Antarctica (which is really hard to predict).  So the potential for even greater—perhaps significantly greater—sea level rise exists.
  • A sea level rise of 2 m will be devastating to Louisiana, south Florida, and coastal North Carolina as this EPA website demonstrates.
  • To the extent that we are already committed to some warming, we are likely already committed to sea level rise.  This study suggests at least 75 cm.

1Vermeer, M and S. Ramstorf (2009) Global sea level linked to global temperature. Proceedings of the National Academy of Sciences

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Posted in climate change science, sea level rise | 1 Comment »

In this week’s issue of Science: Greenland ice loss is accelerating, and now we know why

Saturday, November 14th, 2009

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Greenland and Antarctica are two places that climate scientists are studying intensely because of the potential for significant sea level rise were they to melt. Over the past 20 years, scientists have used a variety of methods to track ice loss, and they have found that Greenland has been losing ice more rapidly over the past decade than it had in the 1990s.  In fact, since 2004, ice loss has accelerated to such a high level that Greenland is now losing about 270 billion tons of ice per year.  Greenland’s contribution to sea level rise has been about 0.13-0.74 mm/yr, or about 4-23% of global sea level rise observed from 1993-2005.

In this week’s issue1,2 of Science (subscription required), Michiel van den Broeke and colleagues used a couple of methods to confirm that this acceleration of ice loss is real and to understand why it’s happening.

The extent of ice in a glacier is like a bank account, but instead of money, we’re keeping track of ice.  When inflows (precipitation = snow) exceed outflows (mostly due to melting and runoff), the ice sheet gets bigger, just like a bank account grows when deposits exceed withdrawals.  We say that there is a positive surface mass balance.  When outflows exceed inflows, then the ice sheet shrinks, and we say there is a negative surface mass balance.

They found that before 1996, Greenland’s ice sheet had a positive mass balance (getting bigger) because precipitation exceeded runoff.   Between 1996-2004, precipitation and runoff both increased, and since these roughly cancel out one another, the ice sheet didn’t change much.  However, after 2004, precipitation stopped increasing while runoff continued to rise exponentially.  Mass balance has been negative for about five years now, with a cumulative mass loss of almost one trillion tons of ice in that span.  Amazing.

The next big question, therefore, is what’s causing precipitation to change? Will it go back up, thereby reversing the ice loss, or will it remain the same or decrease, causing loss to continue accelerating?  Nobody knows at this point.

1van den Broeke (2009) Partitioning recent Greenland mass loss. Science 326:984

2Bowdoin people can access the article here.
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Posted in climate change science, polar ice, sea level rise | 2 Comments »

In this week’s issue of Science: Bangladesh at the front lines of adaptation

Friday, October 30th, 2009

In “Hot, Flat, Crowded—And Preparing for the Worst“,1,2 (subscription required) Mason Inman lays out how Bangladesh is already coping with climate change.

Bangladesh is being hit with multiple kinds of challenges:

  • salt water incursion as sea level rises, affecting water supplies and crops along the coast
  • concentration of rainfall events into fewer but heavier downpours, which leads to both drought in winter months (and failure of crops) as well as severe floods during the summer monsoons that wipe out crops and destroy infrastructure

Some excerpts:

Bangladesh is striving to become a global showcase for climate change adaptation. Earlier this month, its government approved a wide-ranging strategy for dealing with climate change that includes ramping up civil engineering projects to control flooding and protect farmland from rising sea levels. Researchers here are also testing crops that better tolerate floods and drought. Realizing that time-honored approaches to living off the land no longer suffice, Bangladesh has implemented more community-level projects than any other country to gird people for climate shifts.

The World Bank estimates that as much as $100 billion a year is required to prepare people in vulnerable areas for climate change. That’s assuming the world gets its act together to rein in greenhouse gas emissions. If not, says disaster expert Ian Burton of the University of Toronto in Canada, “then the cost of adaptation is going to be enormous.”

It would be interesting for someone to estimate what the more-catastrophic adaptation cost scenarios look like compared to mitigation costs.  This would make it clear what it costs to mitigate now vs. trying to adapt later when it’s more difficult to do so (if at all possible by that point).

We essentially have four choices:

(1) mitigate now, adapt now
(2) mitigate now, adapt later
(3) mitigate later, adapt now
(4) mitigate later, adapt later

#1 will likely be the least expensive option in the long term, and it will help us sustain the fewest impacts in the near term.  It gives us the most flexibility in terms of how we shape the future, and it buys us the most insurance against catastrophic change.  The Stern Review suggested global  mitigation costs of 1-2% world GDP (about $600 billion-1.2 trillion/yr).  That number goes up the longer we wait.  A recent Congressional Budget Office estimate of the Waxman-Markey House bill for a U.S. cap-and-trade program was $22 billion/yr  (roughly the cost of a postage stamp per day for the average American household) by the year 2020.

By eliminating the up-front costs of adaptation, #2 might appear to save money, but if we don’t adapt to change we are already committed to, the costs associated with warming impacts may be large as we lose coastal real estate and farmland, sustain infrastructure damage from more severe storms and flooding, lose crop productivity, and face public health concerns from things like heat waves.  This option is like refusing to pay a few hundred bucks a year for homeowners insurance but then having to pay several hundreds of thousands of dollars to rebuild after a fire.

#3 doesn’t make much sense because we will end up spending twice on adaptation—once to confront near-term changes we are already committed to and once again to deal with (or at least attempt to deal with) conditions getting much worse.  Moreover, mitigating later may be too late to avoid potentially dangerous temperature rise, and it reduces our chances of lowering atmospheric CO2 if climate change is irreversible over hundreds of years.

#4 is truly a losers game.  Ecologically, socially, and economically, it would likely be catastrophic in all terms.

An ounce of prevention may indeed be worth a pound of cure.

Related post:  Climate adaptation: We have no choice, and it’s not enough

1Inman M. (2009) Hot, Flat, Crowded—And Preparing for the Worst. Science 326:662-663.

2Bowdoin people can access the article here.

Posted in climate adaptation, climate change science, food and agriculture, sea level rise | 2 Comments »

In this week’s issue of Nature: Ice sheets thinning

Thursday, October 15th, 2009

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Most people are aware that the loss of ice from land masses like Greenland and Antarctica have the potential to raise sea level.  One of the current concerns about polar ice loss is that as sea level rises, ice loss is accelerating.How does this work?  Much of the ice along the margins of these land masses is what we call “grounded” –it physically stuck on the rough land lying under the coastal ocean.  Imagine a block of ice sitting on a dinner plate, which, in turn, is sitting on something rough like a carpet.  If you tilt up one end of the plate just a bit, gravity causes the ice to slide off until it hits the carpet, where it gets stuck.  Most of the ice block remains on the plate but a bit is wedged (grounded) into the carpet.  This is what basically happens to many of the glaciers flowing off the Greenland and Antarctic land masses.

Here’s the bad news:

(more…)

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Posted in climate change science, polar ice, sea level rise | 1 Comment »

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