28 Dec 2011

2011-2012 Field Season Photos -- collected by Rachel Gesserman

The 2011-2012 POLENET season is underway and off to a great start. This year the team will be working out of McMurdo Station, Byrd Camp, and ALE Union Glacier Camp in order to install 4 GPS sites, remove 13 seismic stations, and maintenance 31 GPS and 22 seismic sites throughout the West Antarctic ice sheet. The sites that are serviced will continue to operate and collect data for another year. Many of the sites will be reached through use of either a Basler or Twin Otter airplane, but this season there is also a skidoo traverse that will travel along the temporary seismic array for up to 2 weeks at a time to remove stations.

 

Skidoo_traverse_photo_credit_dr

From left to right: Cristo Ramirez, Dr. J.P. O’Donnell, Mike Roberts, and Andrew Lloyd preparing to leave on the southbound section of the seismic traverse to remove temporary stations.

Photo credit Dr. Douglas Wiens

___________________________________


 

Seismic_st01_photo_credit_brian_bagley

From left to right: Dr. Douglas Wiens and Mark Whetu helping to load equipment removed from the temporary seismic station ST01.

Photo credit Dr. Brian Bagley

___________________________________


 

Seismic_st09_photo_credit_rachel_gesserman

From front to back: Mark Whetu and Dr. Pippa Whitehouse helping to dig out temporary seismic station ST09.

Photo credit Rachel Gesserman

___________________________________


 

Seismic_st10_photo_credit_betsy_spencer

From left to right: Brian Harrison, Paul Carpenter, Dr. Brian Bagley, Abigail Sussman, and Alex McCorquodale removing temporary seismic station ST10 from the seismic transect.

Photo credit Betsy Spencer

___________________________________


 

Seismic_byrd_photo_credit_paul_carpenter

From left to right: Dr. Douglas Wiens and Dr. Brian Bagley getting ready to dig out equipment and collect data from Byrd seismic station. This station will be revisited later in the season to install a Xeos Iridium modem in order to monitor the station’s state of health and to make it part of the permanent POLENET array. 

Photo credit Paul Carpenter

___________________________________


 

Seismic_siple_dome_photo_credit_pippa_archer

From left to right: Rachel Gesserman and Paul Carpenter beginning to service seismic equipment and collect data from Siple Dome station.

Photo credit Dr. Pippa Whitehouse

___________________________________


 

Seismic_upper_thwaites_glacier_photo_credit_rachel_gesserman

After a long day of digging, the Upper Thwaites Glacier seismic station was partially rebuilt to continue collecting data for another year.

Photo credit Rachel Gesserman

___________________________________


 

Gps_wilson_nunatak_photo_credit_jeremy_miner

Dr. Terry Wilson helps service the Wilson Nunatak GPS station. Besides ensuring that the stations will continue to run for another year, Dr. Wilson also investigated the bedrock at each site she visited.

Photo credit Jeremy Miner

___________________________________


 

Gps_pecora_escarpment_photo_credit_jeremy_miner

Pecora Escarpment GPS station after being fully serviced and left to record data for another year. The station is at the edge of an abrupt cliff, requiring personnel to harness themselves to the rock while working.

Photo credit Jeremy Miner

___________________________________


 

Gps_howard_nunatak_photo_credit_jeremy_miner

Howard Nunatak GPS station after servicing, with both the seismic station and twin otter airplane visible in the background.

Photo credit Jeremy Miner

___________________________________


 

Gps_mt_suggs_photo_credit_jeremy_miner

The Mt. Suggs station is traditionally damaged by wind and bad weather. This year the station had to be rebuilt in order to continue running for another year, which required the replacement of 22 batteries.

Photo credit Jeremy Miner

___________________________________


 

Gps_cordiner_peak_photo_credit_jeremy_miner

 

On top of this quartzite outcrop with a great view of the Duffek Mountains, the Cordiner Peak GPS station was serviced with standard maintenance procedures and left to collect data for another year.

Photo credit Jeremy Miner

___________________________________


 

Gps_haag_nunatak_photo_credit_jeremy_miner

The Haag Nunatak GPS station had 2 destroyed wind turbines. Although only one was replaced, solar power will help to keep it running for another few years.

Photo credit Jeremy Miner

___________________________________


 

Gps_lepley_nunatak_photo_credit_jeremy_miner

From left to right: Mark Whetu and Dr. Eric Kendrick helping to install the Lepley Nunatak GPS station. After four years of trying to build this site and being unable to due to bad weather, the station is finally complete.

Photo credit Jeremy Miner

___________________________________


 

Seismic_and_gps_kohler_glacier_photo_credit_paul_carpenter

From left to right: Mark Whetu and Dr. Eric Kendrick helping to install the Lepley Nunatak GPS station. After four years of trying to build this site and being unable to due to bad weather, the station is finally complete.
Photo credit Jeremy Miner

 

___________________________________


 

 

 

28 Dec 2011

Building an Antarctic Seismic Station -- by Rachel Gesserman

It has been almost 2 weeks since I arrived at Byrd camp in the middle of the West Antarctic ice sheet and our project is off to a great start. The camp itself is pretty remote and home to about 35 people. This includes the scientists working on the POLENET project, pilots from Kenn Borek Air who fly the Twin Otter and Basler airplanes to our seismic and GPS sites, and a hard-working camp staff made up of weather observers, kitchen staff (who happen to be excellent cooks!), and people who do daily tasks around camp. You can probably imagine with only 35 people, the camp is not too big. There are several semi-permanent tents that house the galley, washroom (aka Byrd Bath), science equipment, pilot sleep quarters, and a meeting tent where we can watch movies and hold science lectures. There is also a big plot of mountain tents where everybody else sleeps called “tent city” that is further away from some of the loud, heavy equipment used at camp. 

 

 

Gesserman_blog2_photo1

One challenging aspect of working here is figuring out what GPS and seismic sites to fly to each day. The process is a little tricky because in order to fly, the weather must be good at Byrd camp, at the site we are flying to which can range from 100-600 miles away from Byrd, and at a fuel cache along the route if the plane needs to re-fuel during the trip. If the weather forecast does not look good at any of these places, we cannot fly for safety reasons. Another complication is that the weather in Antarctica is rapidly changing, and the plan that we develop each night for where we want to fly can often change in the morning when the pilots get an updated forecast. This makes for some hectic mornings of getting the right people and equipment organized to fly because the planes can only hold a certain amount of weight and the equipment needed varies from site to site. Since I have arrived we have been able to visit 5 seismic sites. Some are only temporary stations that have been fully removed, and others we are collecting data from and performing maintenance on to keep them running for another year. Although the setup of a seismic site may not look like much on the surface of the snow, inside all the buried boxes is an intricate setup of equipment that will record earthquakes from around the world. The following picture is of a temporary site that we had to completely dig out and remove.

 

 

Gesserman_blog2_photo2

On the right side of the photo you can see a circular red dome. Under this dome is the actual seismometer, which is insulated in a foam box. Burying the sensor helps to keep out a lot of unwanted noisy signal created from wind because it is shielded by the dome and the snow. The sensor is extremely delicate and sensitive, and is composed of three masses in a spring-like system. Each mass is positioned differently in order to record ground motion in the east-west, north-south, or up-down direction. You can’t see it, but there is a cable running from the sensor through the snow and into the orange box on the left side of the photo where it is connected to a piece of equipment called a datalogger. The datalogger records these mass movements as digital data that can be viewed on a computer. Several times a day, the datalogger will transfer the digitized data to a small box called a baler that contains memory sticks. When we visit a seismic site, we can remove the baler to get all of the seismic data recorded over the last year or two. 

 

 

Gesserman_blog2_photo3

But how do we keep this station powered up and running all year long in the harsh Antarctic weather? In the back of the station photo is a set of three solar panels. There is a cable running from these solar panels through the snow and into the orange box where it is connected to a power module. The datalogger that I discussed before is also connected to this power module, along with 10 car batteries. During the 6 months of sunlight in Antarctica, referred to as austral summer, the solar panels charge the car batteries that provide power for the station. During austral winter when there is no sunlight, the batteries provide enough power to keep the station running. Just for comparison, when we install sites in other parts of the world we only need 1 car battery because the solar panels receive sunlight every day. You also might notice an antenna on top of the solar panels. This is a GPS antenna that is extremely important because it allows the station to know the exact time. The timing needs to be accurate to about 1/100 of a second; otherwise our data will be useless because we won’t know how long it takes seismic waves from an earthquake to reach the station. But when everything is set up correctly and functions without a problem throughout the year, we get excellent data like this signal from the March 11, 2011 Japan earthquake recorded at one of our seismic sites called Siple Dome. You can see how the earthquake looks on the up-down, north-south, and east-west components of the seismometer. We can use this data along with data from many other earthquakes around the world to make a map of the structure of the earth beneath Antarctica.  Here’s hoping that the rest of our data collected this season looks as good! 

 

 

Gesserman_blog2_photo4

28 Dec 2011

Traveling to Antarctica - by Rachel Gesserman

After boarding three commercial flights, one Air Force C17, and traveling for about 33 hours, I finally arrived in one of the most remote places on Earth – Antarctica. For the next five weeks I will be based out of a small field camp in the western part of the continent called Byrd (named after the great Antarctic explorer Richard Byrd) to begin working on part of a project called POLENET, or the Polar Earth Observing Network. This particular project involves collecting GPS and seismic data that will help answer questions about the structure of the continent, the behavior of the Antarctic ice sheet, and how these may be related. My role will be to work primarily with the seismic instrumentation in West Antarctica, helping to collect data and perform any necessary maintenance on already installed instruments.

 

Gesserman_blog2_photo1

 

You may be thinking this all sounds interesting and exciting, but how exactly do earthquakes help us learn about the structure of Antarctica? Seismology is not strictly the study of earthquakes. Seismologists also use seismic waves from earthquakes to study the Earth. Just like an X-ray will expose details of the body invisible to the human eye, the behavior of seismic waves expose details of the Earth from the shallowest crustal layers all the way down to the core of our planet. When an earthquake occurs, there are several types of seismic waves that propagate through the Earth and are recorded at seismic stations. These waves move at different speeds depending upon what type of material they are traveling through. For example, a seismic wave will slow down when traveling through warmer layers. Seismologists can figure out how fast a wave is moving through different parts of the Earth by looking at the wave arrival time at a variety of seismic stations. The denser the instrumentation, the more detail can be observed about the underlying structure. By installing seismometers in Antarctica, we are able to figure out how the crust differs throughout the continent. 

 

 

Why do we care? It is important to understand the interaction between the ice sheet and the underlying bedrock. Think about placing a heavy weight on top of a thick sponge. The weight will create a depression in the sponge. But what happens when you remove this weight? The sponge will bounce back to its original shape. The Earth’s crust behaves in a similar way, but areas of warm, weak crust depress much more than areas of strong, rigid crust when put under the same weight. As the ice melts and relieves some of the weight on the continent, the crust begins to move vertically like the sponge and is detected by the GPS instrumentation. By combining seismic data about the type of crust beneath the ice with the GPS data showing the amount of vertical motion each year, a more accurate calculation can be made of the amount of ice that is melting and how fast it is happening. This rate of melt has a direct impact on sea-level change. In a time when global warming is a hot topic in science, it is important to figure out how the polar regions of the world are being affected and what that may mean for the future.

 

 

 

Before I can travel to Byrd field camp, there are a few things I need to do to prepare for the extreme environment I’ll be living in. I have spent this past week at the US base McMurdo going through different training courses, including a snow school that is referred to around here as “Happy Camper”. This hands-on two day training began by re-locating to the outskirts of McMurdo, conveniently far enough away to discourage those who may have thoughts of walking back to the cozy base. Throughout the course we learned skills like how to use camping stoves, set up tents, build snow walls to shelter our tents from the wind, build survival trenches for emergency situations, and how to operate a high frequency radio which we tested by calling the South Pole station. Although some of the more adventurous people chose to sleep in their survival trenches, I chose to stay in a tent and learned some good tricks for keeping warm in the process. By putting two bottles filled with hot water inside my sleeping bag, I was very comfortable throughout the night and could also keep my hat, gloves, and neck guard warm for the next morning. To wrap up the training we ran through some scenarios, including how to find a lost team member in the event of a white-out. This involved wearing white buckets on our heads so we were not able to see anything, which I’m sure is always entertaining for the instructors. The camping experience was surprisingly very enjoyable, especially because I had the opportunity to meet a lot of other scientists and learn about some interesting projects taking place in Antarctica. In the end I’d say I was a very happy camper and am ready to head to Byrd! 

 

 

Gesserman_blog1_photo2

20 Jan 2011

Podcast #8: Progress and Life from Byrd Camp

Join us for the unprecedented! Three planes in one day! Check out our latest progress across West Antarctica and get a taste for life here at Byrd Camp.
To see a higher resolution version and download this podcast, please visit http://polenet.org/education/2minutes/

(download)

20 Jan 2011

Podcast #7: Two Weeks into Byrd Camp

After two weeks at Byrd Camp, we've made it out to a few key GPS and Seismic stations in West Antarctica. Come along on our latest adventures!

(download)

23 Dec 2010

Halfway Through the Season!

Current Location: Byrd Camp, West Antarctica
Current Weather: -15º C. Winds at 11 knots.

Christmas decorations are often tucked away behind cellar doors. I remember digging out the tree, the ornaments, the lights-- a responsibility that was inevitably passed on to me based on the fact that I was small enough to shuffle around the cramped space.

Here at Byrd Camp, holiday decorations transpire not from dark, shallow storage rooms but from the creative energy that pulses through camp. A Christmas tree is formed out of metal banding and bamboo tent stakes. Ornaments are cut out from empty Coca-cola cans, toilet paper tubes, paper, string, duct tape, and whatever other supplies can be foraged.

Last week, we had the unprecedented! Three planes were at our disposal- two Twin Otters and a Basler. The elusive Basler flies around the continent and is much coveted by science groups for its range and cargo capacity. We were incredibly fortunate to have it based at Byrd for a whole week. There were days when weather was open at three of our sites and we scrambled to split our team, our tools, and our gear into three successful missions per day.

Because of this impressive availability of planes and a few days of good weather, we’ve been able to reach a number of sites, including a brand new installation at Bennett Nunatak. Also on our list were Kohler Glacier, Clark Mountains, Mount Carbone, Siple Dome (Thanks Paul Winberry!), Iggy Ridge (Thanks Dylan Taylor!), Fallone Nunataks, Mount Howe and 8 sites that lay across the seismic transect.

Dec_map

To explore an interactive version of this map, visit http://polenet.org/livefeed

The weather continues to warm up as the summer season progresses, with temperatures staying in the negative teens (Celsius). As the heat continues to be trapped within my sleep tent walls, whatever is not covered by my sleeping bag or pile of clothes gets melted, and I wind up with a ledge to sleep on, surrounded by an expanding hole underneath the floor. Eric Kendrick’s thermometer read 87º F inside of his tent a couple of days ago when he woke up so over-heated he couldn’t get back to sleep! It’s certainly quite a different story than earlier in the season when a hot water bottle was your only hope for staying warm enough!

We were sad to say goodbye to eleven of our team members last week as they left camp on an LC-130 Hercules. Some headed back to McMurdo to catch a flight to Union Glacier camp, which sits about 690 km grid north or due east of Byrd Camp. POLENET has 6 sites to be reached from Union Glacier. We’re excited to see what they can accomplish before they leave on the 4th of January!
Alas, all of our planes have left us today to head back to McMurdo for maintenance. The upcoming Christmas weekend is giving us a chance to catch up on planning, organize the overflowing piles of paper and gear that have taken over our tables, and prepare for the second half of our season. It’s also a time to connect with everyone here around camp, eat good food (freshies arrived today!), share stories, go sledding on the berm, and take a breath to appreciate where we are. We wish all of you happy holidays!

-megan

9 Dec 2010

Weather, Site Visits, and a Farewell to the Piggies

“Nothing new at Byrd- cloud free and light winds, but expect things to change around 12 Z, when winds and blowing snow will pick up.”

After a few days of absolutely gorgeous weather, these were the words we heard this morning when calling into Charleston, South Carolina for our weather outlook.

Charleston, South Carolina? Yep. Who knew that a weather office so far away could have more useful information than we have here? Because our bandwidth is so limited, we rely on Paul and Michael at SPAWAR to give us the bigger picture of the weather each day at 0100 and 1100. From there, we map out chunks of good weather and bad weather.

10_dec_pm_low3

Once we get an idea of what might happen weather-wise, we send in flight priority requests to Fixed Wing Operations in McMurdo Station, which allows us to receive full weather forecasts for up to 5 locations. With about 40 sites to visit, it’s a constant strategy game we play with weather, fuel caches, site priorities, and number of hours in a day. 

Fortunately, weather has opened up in the last few days and we were able to visit a number of seismic transect sites close to Byrd, as well as GPS and seismic sites at Mt. Sidley, Whitmore Mountains, and WAIS Divide. We are waiting for clouds to clear off from the coast so we can reach some of our sites that lie grid west of Byrd. Unfortunately, with the summer temperatures warming up, fog from the surrounding ocean tends to leach onto the continent.

Getting to Mt. Sidley was no small task. Large thermal uplifts meant that the pilots couldn’t land where they have landed in past visits to the site. After circling 10 times, a somewhat nauseous and eager team was happy to deplane and get to work. They had a long 1.3 mile hike across a boulder-strewn ice field up the side of a volcano, following a flow band in the ice. The icy path allowed them to drag about 250 pounds of gear on sled to repair the station. The Antarctic wind had done its best, shattering 3 of the 4 solar panels. Imagine! Cobbles being thrown up 5 feet in the air and smashing into these panels! That’s an intense wind. Two wind turbines were destroyed. The MET pack had launched about 45 feet away by the wind. The cables had ripped out of the box. There must have been a mega storm out there over the winter! The team was able to collect several months of data and repair the site. 

At the Whitmore Mountains site, which lies further south towards the pole, the team encountered colder temperatures but clear skies. They found the GPS recording data, but one of the solar panels had been shattered and the two wind generators were broken. The lack of communications with the site had been due to a modem problem. Seth and Jamey made all necessary repairs and planned upgrades. The seismic team repaired the seismic station, replacing two broken solar panels. It is now recording data. 

To see an interactive map of our sites, check out http://polenet.org/livefeed.

A few LC-130 Hercules flights into Byrd have brought some much needed barrels of fuel, which are being cached at strategic fuel sites so our Twin Otter can reach further destinations. The Herc also brought us a welcome load of fresh carrots, celery, onions, eggs, and pears! Unfortunately, no chairs arrived, so Mark, one of our mountaineers, rigged up some stools out of spare particle board. When there’s no Home Depot or Target around, it’s amazing to watch how creative people get! String, duct tape, cardboard boxes, and leftover food containers can transform into all kinds of useful things!

We have been operating 24 hours a day, with two flight crews who have tremendous skill in flying in such extreme conditions. Landing at sites where no one has ever been before, without runways, on snow, with variable lighting, and constantly changing weather- the expertise of our pilots is invaluable. In about 6 days, one crew will move to Union Glacier Camp, and we will be operating single-shifts out of both Union and Byrd.

We are also saying goodbye to some of our friends here at camp, fondly named “The Piggies.” They are spending a couple weeks traversing across the snow to Pine Island Glacier (PIG) where they will be setting up a site for future field work next year. We have watched them assemble their sleds and supplies, and wish them all the best! Look out for crevasses and take care of each other out there!

-megan
2 Dec 2010

Greetings from Byrd Camp!

Current location: Byrd Surface Camp, West Antarctica (80º South)
Current weather: Winds at 16 knots. -25º C

The wind that greeted us at Byrd Surface Camp on Monday afternoon was a true Antarctic welcome to our deep field camp! Ferocious and unrelenting, we stashed our gear in the tents and enjoyed a warm meal prepared by our favorite chefs- Robert, Thomas, and Patrick, who manage to do brilliantly tasty things with frozen, canned, and powdered food, with a smattering of freshies that come in sporadically.

Eric Kendrick and Guy Tytgat managed to reach Lonewolf Nunataks- about 475km from McMurdo earlier this week. Once at the GPS site, Eric discovered a broken frame, but all equipment was still operating. He was able to rig some temporary repairs that will keep the station going until they can return to complete more permanent repairs. Guy upgraded the seismic station box to one with more insulation to keep the temperature above -50º and replaced a small GPS antenna, so all in all, it was a long, but successful day.

Now that most of the crew is gathered at Byrd, we’re busy testing equipment, making flight plans, organizing work flow and logistics, and waiting for our Twin Otter and Basler aircraft to arrive in camp. For the past few days, weather has been bad between here and McMurdo, so we continue to wait for that perfect weather window.

We have arrived at camp about a month earlier than last season, which means it’s a lot COLDER than we remember! We’re at the tail-end of Spring, with warmer temperatures on their way in a few short weeks. The put-in crew arrived about three weeks ago, and with heroic efforts, managed to dig out all of the tents and gear that had been stored on high berms from last season. They have epic tales of surviving condition one storms to retrieve their sleep kits-- a few hundred meters can become almost insurmountable when you can’t see your hand in front of your face and the winds are strong enough to blow you off course!

Camping in Antarctica certainly has its drawbacks. Sleeping in a bright yellow tent with 24-hour sunlight can be a challenge. Waking up in the morning and having to climb out of your cozy sleeping bag can be a psychological feat of some magnitude. But, it has its good points as well! Just think: No cell phones, no traffic noise, no spiders, no flies, no bears! No animals at all, really. Although a few lost Snow Petrels (small white sea birds) managed to wander past camp-- we all ran outside and watched in amazement.
The other plus about camping in Antarctica is that you’re surrounded by people whose adventurous streak is seasoned with enthusiasm, hard work, and a willingness to take things as they come, go with the flow, and have fun with life. It’s a great place to get this project moving!

-megan

25 Nov 2010

Video Podcast #6: The start of a new field season!

(download)

Welcome to the 2010-2011 POLENET Field Season! We're just getting geared up to head to Byrd Surface Camp and are attempting to reach 40 GPS and seismic sites located across the continent. Come along with us! Check out our new video podcast and stay tuned for the next one from Byrd Camp in West Antarctica!

Happy Thanksgiving everyone!

P.S. You will all be pleased to know that Frosty Boy has recovered and has rejoined us in the galley!

19 Nov 2010

Frosty Boy, We Send Best Wishes

Current location: McMurdo Station, Antarctica

Current temperature: -9º C  Windchill: -14º C

 

While this blog is in place principally to provide the ins and outs of POLENET field operations, I would like to diverge for a moment and tell the story of one very eventful lunch session in the McMurdo Station galley.

 

But first, we must go back to 1960s Australia and New Zealand-- to the origins of one of McMurdo’s very close friends. The affection and loyalty towards this particular friend is lasting and sincere, and for many years, its return has been one of ever-flowing creaminess.

 

Creaminess? Yes, I am talking about Frosty Boy, McMurdo’s soft ice-cream dispenser.

 

Those who have traveled to McMurdo before will know the meaning of Frosty Boy. In a place that is constantly changing- the weather is never certain, people fly in and out all the time, friends switch from day-shift to night-shift, beer runs out, ice turns to water turns to ice again. Through it all, Frosty Boy is there, a constant and comforting friend.

 

So it was quite alarming when, in the middle of a very busy lunch (as McMurdo is quite crowded in the middle of the summer season), our poor friend let out a loud and lasting screech that silenced the galley and made even the people down the hall turn their heads in amazement. The poor girl who happened to be the last to turn the handle stood wide-eyed, unsure how to proceed. “Unplug it!” someone yelled.

 

Something was unplugged from the wall, and the screeching came to a faltering, miserable stop, and a hush fell through the room, as we all realized that this was something serious.

 

Saddened at the site of our wounded friend, we turned back to our meals, only to be startled minutes later by a burning rubber smell and smoke, followed by further screaming from the depths of Frosty Boy’s soul. A group of people rushed to pull Frosty Boy from his nook, reaching back to unplug him from his last link to life support.

 

If we didn’t believe it the first time, we sure believed it now. Frosty Boy was down.

 

There are many theories as to why this happened. The old ice cream machine from South Pole, aptly named “Frosty Girl” was recently transferred to McMurdo. Perhaps Frosty Boy felt his territory impeded upon?

 

Or take the most recent flavor that was pumped through his veins: Strawberry Balsamic. Maybe it was just too much.

 

But as we watched our friend being gutted and closed down for the repair shop, we took refuge in the final words of our Dining Assistant: “He’s come back from worse.”

 

While we wait for Byrd Camp to open, we continue to fly to stations close to McMurdo when weather is good. And we send our best wishes to Frosty Boy during his eminent recovery.

 

-megan

 

Frosty_boy_berg

 

polenet's Space

POLENET is a global network dedicated to observing the polar regions in a changing world.
The project focuses on collecting GPS and seismic data from autonomous systems deployed at remote sites spanning much of the Antarctic and Greenland ice sheets. GPS and seismic measurements together provide a means to answer critical questions about ice sheet behavior in a warming world.