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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 »

AGU conference highlights

Sunday, December 20th, 2009

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There was a break in the blog action last week because I was attending the American Geophysical Union meeting in San Francisco.

As usual, this was a great meeting with over 16,000 climate and earth system scientists from around the world gathering to share the current state of their research.

Some highlights:  Richard Alley (world-renowned climate scientist at Penn State) gave an interesting talk linking CO2 and climate change across multiple geological scales.  It’s a 50-minute version of my Earth Climate History course I’m teaching next semester at Bowdoin.

There were other keynote addresses, but Alley’s was the most appropriate for a general audience.

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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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Climate science and moving beyond hackergate

Sunday, December 6th, 2009

By now, everyone has heard of the hacked emails from the British Climate Research Unit (CRU) at East Anglia University. The play-by-play has been getting a lot of press, especially at Dot Earth and Climate Progress.  Rather than focus on the specifics, I want to help us keep focused on larger issues, which I think is useful for getting past the heated rhetoric.

Yesterday, Bryan Walsh ran a story, As Climate Summit Nears, Skeptics Gain Traction in Time Magazine in which the following passage appeared:

Even a small amount of doubt is enough to shatter consensus. That is why a number of researchers have suggested in the wake of the CRU e-mail hack that climate scientists be more open with their data and engage with critics in the future. “Climate McCarthyism” — as Ted Nordhaus and Michael Shellenberger of the Breakthrough Institute have called the knee-jerk attacks by some climate-change advocates on those who deviate from the green mainstream — must stop. That may not seem fair — industry groups have played dirty for years smearing climate scientists — but researchers will need to be above reproach. “Scientists need to consider carefully skeptical arguments and either rebut them or learn from them,” wrote Judith Curry, an atmospheric scientist and climate researcher at Georgia Tech, on the blog Climate Audit.

There are several things to consider:

The scientific process is a powerful tool—in many ways, the most powerful tool we have.  All ideas should be allowed at the table and should be investigated thoroughly.  Yes, even the ideas of climate skeptics.  The notion that scientists might have attempted to short circuit the peer review process is unfortunate. This should never happen.

However—and this is an important point that has not been stated strongly enough—when a fair peer review process rejects ideas for not standing up to intense scrutiny, as determined by several sources of empirical observations and models, it’s time to move beyond the false ideas for the sake of clarity and efficiency.  Climate skeptics and warming advocates alike who lose on the battlefield of peer review need to own their loss, suck it up, and move on.  Returning to the table is fine, but do it with new ideas that better help us understand the way the world works, rather than trotting out retreads or, worse, advancing an agenda.

I tell my students that the outcome of science isn’t meant to be fair.  However, the process of science is fair.  At the starting blocks, it accepts all ideas and sifts through them one by one to see which ones stand the test of scrutiny (data and models and other lines of evidence) and which ones don’t.  The ones that don’t are discarded to the dustbin of history.   The ideas that survive get to live another day until subject to refined analysis and new data, models, and ways of thinking.  Over time, if they continue to survive, they become generally accepted ways of describing our world.  Much of what we know about climate warming, such as the  role of greenhouse gases in causing warming, fits this bill.  Of course, something may come along that could revolutionize conventional wisdom—Einstein did that to Newtonian mechanics with his theories of relativity—but until that happens, scientifically based conventional wisdom that has withstood the test of time is simply the best process we have at getting closer to the truth on climate warming science.

Problems arise when people conflate outcomes and process—equating, for instance, a bad outcome (rejected idea) to an unfair process.  This can lead to a rejection of science as a a way of knowing, and that’s unfortunate.  People don’t have the choice of rejecting the scientific method simply because they lose.  That’s the game of a poor loser.  The challenge is for them to come back with a winning idea.

It’s all too easy for climate science to become politicized.  Everyone knows this.  With regards to skeptics—contrarian for the sake of contrarianism.  With regards to warming advocates—overly dismissive of alternative viewpoints.  At that point, science crosses the threshold to ideology, which has no place in the peer review process. Fortunately, ideology seldom lasts long in a well-oiled peer-review meat grinder.

So why don’t I worry?  Because I return, over and over, to a singularly powerful idea:  In the end, a fair peer review process will lead us closer to the truth.   The furnace we call the climate warming debate is blistering.  This is why we must make sure the crucible of a fair review process is strong enough to withstand it.  And so far the peer review process most likely has been fair.   There are too many independent research groups studying climate change, involving tens of thousands of scientists worldwide, who are reaching the same basic conclusions about warming.  It’s simply impossible for a conspiracy to ever grow that big.

What we need now more than ever is for both sides of the climate debate to consider all ideas and for the losers (of a fair process) to own their loss.   Sure, it’s a high-stakes game, and nobody likes to lose.   But some will.  The question is whether the losers will continue by pushing an agenda rather than useful ideas.   History will be a harsh critic of those who do.

Posted in climate change science, climate skeptics deniers and contrarians | No Comments »

Climate warming worse than previously thought because plants become nutrient starved?

Tuesday, November 24th, 2009

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The biosphere—the sum of all living organisms on earth—has tremendous influence on our atmosphere and climate.   One active area of research is how the biosphere might respond to rising atmospheric CO2 and temperatures and how this change, in turn, can influence (“feed back on”) climate change.

There are several reasons to believe that the biosphere will generate positive feedbacks on climate (i.e., things that cause warming to accelerate) as a result of warming.  For instance, rising temperatures can

  • increase soil organic matter decomposition, causing the release of CO2 to the atmosphere, which causes more warming;
  • increase permafrost thaw, thereby releasing old carbon and methane bubbles previously locked in a deep freeze;
  • cause treeline to advance northwards.  As conifer trees move north, the formerly tundra landscapes darken as evergreen trees replace open expanses of snow in the winter.  This causes regional climate to warm.  In fact, one study1 showed that without its boreal forest tree cover, Canada would be about 5-10 degrees C colder in the winter and spring!

Positive feedbacks are a nightmare scenario because they mean that climate can warm faster than we currently expect, indicating that we have even less time to deal with it.  This is part of the reason why scientists are beginning to sound the alarm that CO2 is rising faster than expected and the impacts warming are happening faster than predicted.

There are also reasons to believe that the biosphere will generate negative feedbacks on climate (i.e., things that cause warming to slow) as a result of rising CO2.

  • For example, because CO2 is limiting to photosynthesis, rising CO2 is causing plants to grow better.  This removes CO2 from the atmosphere and can slow warming.

In a forthcoming article2 in Geophysical Research Letters, Wang and Houlton analyze this negative feedback, and offer some sobering news about it’s ability to slow warming.

Specifically, they argue that most models used in the IPCC assessments do not take into account the fact that nitrogen becomes limiting in soils as plants grow more.  Nitrogen is one of the most limiting nutrients to plant growth, which is why it’s often the #1 ingredient in fertilizer used on crops and lawns.  Many field studies (example) have shown that trees grown in elevated CO2 experiments eventually stop growing any better than those in ambient air after a few years because soil nitrogen runs out.

Without nitrogen limitation, the IPCC models allow plants to grow more and remove more CO2 from the atmosphere via photosynthesis than they do in reality.   This means that forests may not be as strong a carbon sink as we suspect, and we are therefore underestimating the rate of CO2 rise and magnitude of warming—maybe by as much as 0.5 degree by 2050 and 1 degree by the year 2100.

1Bonan, G. et al. (1992) Effects of boreal forest vegetation on global climate. Nature 359:716-718.

2Wang, Y. and B. Houlton (in press). Nitrogen constraints on terrestrial carbon uptake: Implications for the global carbon-climate feedback. Geophysical Research Letters
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The cost of warming—393,000 increased deaths from civil war in Africa by 2030

Tuesday, November 24th, 2009

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A new article1 by Marshall Burke and colleagues this week in the Proceedings of the National Academy of Sciences (open access) explores the relationship between climate and conflict.

Their argument is that historically warm years have correlated strongly with increased warfare in Africa.  Based on this relationship and GCM projections of African climate, they forecast a 54% increase in armed conflicts by the year 2030, resulting in 393,000 additional battle deaths.

One might wonder about precipitation changes associated with climate warming—Do they alter this result?  Short answer:  No. The temperature-conflict model was robust regardless of whether or not precipitation was included.

1Burke, M.  et al. (2009) Warming increases the risk of civil war in Africa. Proceedings of the National Academy of Sciences 106:20670-20674.

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Earth’s oldest organisms reacting to climate warming

Monday, November 16th, 2009

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Imagine being a tree growing incredibly slowly for the last 4,600 years and then suddenly experiencing a significant growth spurt in the last half century.

In this week’s early edition of the Proceedings of National Academy of Sciences (open access), Matthew Salzer and colleagues report that several stands of bristlecone pines growing at treeline in California have experienced such a jump.

The growth of trees at high elevation is often limited by temperature.  There has been debate among plant physiological ecologists as to why this is the case.  It appears that tissue formation is limited in this species by very cold conditions. By warming up climate, trees are able to create wider growth rings. The result is faster growth and a great paleothermometer to help us further document the significance of modern climate warming.

The researchers also factored out possible effects of precipitation and CO2 fertilization, indicating that the growth increase, unprecedented over the past several millennia, is likely caused by temperature.

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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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Climate Literacy—A new blog

Friday, November 13th, 2009

climateliteracy (Custom)Here’s a new blog to keep an eye on.  It’s called Climate Literacy by Mark McCaffrey in the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.

It’s aims:

Because climate science is inherently interdisciplinary, it can fall through the cracks in traditional science education. Students sometimes graduate from high school or even college without learning climate basics. Climate literacy is aimed at helping address these gaps.

Related posts:

Posted in climate change science, environmental literacy | 1 Comment »

Kilimanjaro ice loss likely due to warming, not drought

Tuesday, November 3rd, 2009

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When I was in grad school, I saw a talk by Lonnie Thompson—a paleoclimatologist who climbs the most remote mountain peaks in the world to take ice cores from glaciers.  Over the past several decades, he has used ice layers to generate historical records of environmental change at low latitudes (near the tropics) as a useful comparison to the ice core work at Greenland and Antarctica.  Hands down, Lonnie has some of the most amazing and treacherous expeditions in the world.  If you ever get a chance to see a photo of how they traverse ice crevasses with field gear, it’s amazing.

In a forthcoming article in the Proceedings of the National Academy of Sciences (open access), Thompson’s team climbed Mt. Kilimanjaro in Africa to assess the rate of ice loss on this mountain.  Kiliminanjaro has been losing significant ice for decades–as much as 80% gone by 2000.

Although this seems like obvious evidence of climate warming, there has been considerable debate about the additional role of drought.  Glacier growth is controlled by precipitation, and if east Africa has experienced drought for the last few decades, this might also be affecting the Kilimanjaro glaciers through (1) reduced snowfall and (2) increased solar radiation.

The team collected 6 cores dating back 11,700 years ago.  What did they find?

  • Based on the rates of ice loss, they expect the disappearance of all Kilimanjaro ice between 2022-2033.
  • 26% of the ice was lost between 2000-2007.
  • Based on meteorological data, they argue that there is a distinct warming trend in the region over the 20th century but not clear evidence for extensive droughts.
  • Since glaciers are also being lost in other low-latitude sites (Indonesia, Tibet, and the Himilayas), this suggests a global warming effect rather than regional droughts.
  • Also, the Kilimanjaro ice survived a past drought around 4,200 years ago that lasted for almost 300 years, and the ice has persisted for the last 11,700 years, suggesting it has weathered many past climate changes during our current, warm interglacial period known as the Holocene.
  • All of these lines of evidence suggest that less cloudy and drier conditions are not likely driving ice loss.

This is another amazing example of something that is going extinct in our lifetime because of global change.

photo credit: http://www.flickr.com/photos/appenz/ / CC BY 2.0

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