How Satellites Monitor Earth’s Ice from Space

Introduction

There is approximately 33 million km³ of ice on Earth, with much of it contained in the huge ice sheets of Greenland and Antarctica. It is estimated the Earth is losing a trillion tonnes of ice per year. How do we know this? It’s all thanks to our eyes in the skies: satellite missions. This video explains how satellites are able to monitor the Earth’s ice from space using remote sensing technology called altimetry.

The Importance of Satellite Monitoring

“Before we had satellites looking at polar regions, we really didn’t know very much about them at all, apart from what people collected when they visited them. They are so huge we can’t possibly hope to measure all of them on foot, but when satellites came around we could see that they were really useful for looking at other parts of our planet and eventually we persuaded space agencies to launch missions over the poles. And now we have a really clear picture of what’s happening there, a very different picture to what people thought before we had those measurements.” – Professor Andrew Shepherd (CPOM Director)

Why Ice Sheet Monitoring Matters

It’s vital for scientists to have a strong grasp of how much ice there is on Earth and how much is melting, for a variety of reasons:

Sea level rise: The ice sheets hold vast amounts of water, which goes into the ocean when it melts, contributing to global sea level rise.
Climate indicator: Ice sheet mass balance is a key indicator of climate change, and monitoring changes in the Earth’s ice helps scientists understand how the climate system is changing over time.
Ocean circulation: Melting ice sheets can affect ocean circulation and global climate patterns.
Future planning: Accurate data on ice sheet mass balance improves climate models and projections so policymakers can prepare for future scenarios.

How Altimetry Works

“We have a sensor that’s mounted on either an aircraft or a satellite, which is a downward-looking instrument. It fires a pulse of light—electromagnetic radiation—down towards the surface of the Earth. That pulse is reflected back from the Earth’s surface and then received again by the satellite. Because we know the speed of light, we can convert that to the time it’s taken for that pulse to travel to the surface and back again.

We also know the altitude of the satellite because we know the orbit really precisely, and we know the mean sea level. So, you can combine all that information to work out the height of the surface that the pulse is reflected from.” – Dr Tom Slater (CPOM)

Types of Altimetry Technology

There are two different types of altimetry technology used by CPOM scientists to estimate the height of ice sheets: radar and LiDAR. Although they both measure the same thing, they have different attributes:
Radar altimetry: Radar signals can pass through clouds, but they also penetrate the ice sheet surface, and adjustments have to be made to compensate for this effect.

Laser altimetry: Laser signals reflect on the actual surface but cannot record when clouds are present.

Key Satellite Missions

NASA’s ICESat-2 uses laser altimetry
ESA’s CryoSat-2 uses radar altimetry

In 2020, the Cryo2ice campaign, an ESA-NASA partnership, adjusted CryoSat’s orbit to synchronize with ICESat-2, which has enabled near-simultaneous collection of radar and laser satellite data over the same areas.

“These two missions are very important because they’ve given us proof that ice is melting around the world. By using two different methods and techniques, we are able to combine them, and they show that the Earth’s cryosphere is changing. This is happening because of global warming.” – Dr Tommaso Parrinello (ESA)

Comparing Different Altimetry Methods

“Recently in CPOM, we did some work comparing different types of altimetry in Greenland—radar altimetry and laser altimetry from two different satellites: CryoSat-2, which is a mission run by the European Space Agency, and ICESat-2, by NASA. We found that these two altimeters agree really well over the same period of time in the same locations. That’s a really important finding because it means we can use them together to learn even more about the ice sheet.

One of the benefits of altimetry is its higher spatial resolution compared to some other geodetic techniques that we use to monitor changes in ice mass. For example, it can tell us about how individual glaciers are responding to climate change. They all respond differently depending on how they interact with the Earth system, and that’s where altimetry is really important.” – Dr Tom Slater (CPOM)

Global Ice and Sea Level Rise

“In Greenland and Antarctica there are the two big ice sheets, and we also have land ice in mountain glaciers all across the world. When that ice is melting, it directly contributes to rising sea levels. This is really important because around 40% of the global population lives in coastal areas.” – Dr Inès Otosaka (CPOM)

“We’re interested in a lot of different properties of the ice sheet and how they behave, and we can combine all these different observations. Altimeters tell us about changes in ice elevation and volume, but we want to know the mechanisms behind those changes. So we would use other observations to help us attribute the changes. That could be changes in ice flow, changes in snowfall on top of the ice sheet, or an increase in surface melting.

Altimetry can tell us something about all those different behaviours.” – Dr Tom Slater (CPOM)

Other Methods: Gravimetry

Changes in ice sheet mass can be observed using altimeters, which measure the height of the ice sheet surface, but they can also be measured through gravimeters like the GRACE satellite, which are able to weigh the ice from space by monitoring changes in the Earth’s gravity caused by movement of melting ice that has entered the ocean.

It’s also possible to contrast satellite measurements of ice flow with climate model estimates of how much snow is falling onto the ice sheet.

The Ice Sheet Mass Balance Intercomparison Exercise (IMBIE)

The Ice Sheet Mass Balance Intercomparison Exercise, or IMBIE, led by CPOM, is a community effort to bring together multiple estimates of ice sheet mass balance to gain the most robust assessment of how much ice is on Earth, how much is melting, and what this means for sea level rise.

“We measure how much ice the Greenland and Antarctic ice sheets are losing. When we aggregate lots of different satellite estimates of ice sheet mass balance, we can actually try to reduce our uncertainties in our observations and then produce a robust dataset that can be used, for instance, by the ice sheet modelling community so that they can calibrate the ice sheet models and reduce uncertainties in their projections of future sea level rise coming from the ice sheets.

Within IMBIE, we track ice losses from Greenland and Antarctica. Because we do that from satellites, we can now have a monthly record of how the ice sheets are changing. We don’t really get big surprises because we have the capability of monitoring month by month what the ice sheets are doing, which is very valuable. We can see whether the accumulation that we get in winter is enough to offset the melting that we have in summer, for instance. We have found that Greenland and Antarctica have been losing mass more rapidly over the last decade.” – Dr Inès Otosaka (CPOM)

Ground-Truthing and Validation

It’s important to gather data on the ground to complement the altimetry data we gain from satellite missions, but also to calibrate the information to improve accuracy as well as to better understand the differences between radar and laser data.

“Sometimes we have the chance of actually going to those places through field campaigns. We can have airborne campaigns where we use similar instruments on aircraft as we use on the satellites to make sure that what we see on the ground is actually what we measure from satellites.” – Dr Inès Otosaka (CPOM)

The Big Picture: Models and Observations

“Climate science is huge. Everyone can’t do everything, so we all have our specialties. But we’ve found that climate models and satellite data go hand in hand with each other. In fact, everything we’ve learned about climate change is thanks to those two things. We wouldn’t know what we know today if we didn’t have satellite measurements, and we wouldn’t know about the future if we didn’t have models.

But the models need to be grounded in reality, and that’s what the satellite observations provide.” – Professor Andrew Shepherd (CPOM)

BISICLES Ice Sheet Model: Transcript

With Dr Tom Mitcham and Dr Sammie Buzzard.

What is CPOM?

CPOM, the UK Centre for Polar Observation and Modelling, is a group of scientists from six universities in partnership with the British Antarctic Survey. We use observational data primarily from satellite missions and combine this with advanced numerical modelling capability to create the clearest possible picture of polar regions. Our science helps enhance understanding and prediction of the environmental change taking place on our planet, to help society prepare for the global impacts of climate change.

Understanding Ice Sheet Modelling

“Modelling is creating simulations of processes that are happening on our planet. We take mathematical and physical equations, often using computers to solve them, and the outputs allow us to learn more about what’s happening on our planet and the reasons behind it.”

What is BISICLES?

“BISICLES is an ice sheet model that was developed partly by CPOM researchers and allows researchers to simulate ice sheets and to look at how they might change in the future. It’s computer software that allows us to solve the equations that we think best describe how ice flows and moves in response to different forces.

BISICLES is particularly special because it allows researchers to look with really high resolution at the most important parts of the ice sheet. Antarctica is really big, and creating a simulation of it takes a lot of computing power. What BISICLES does is concentrate the modelling on the parts of the ice sheet that we think are most important for the processes that might be changing as the ice sheet evolves.”

Why High-Resolution Matters

The margins of an ice sheet are complex and can be subject to rapid and unpredictable change. The grounding line is where the ice sheet on the bedrock of land meets the sea, often in the form of colossal ice shelves that float on the ocean. Rapid changes at the margins of ice sheets can lead to significant ice loss and sea level rise. High resolution modelling capability is required to create the most robust projections of ice sheet dynamics and sea level rise.

The Development of BISICLES

“It became apparent in the late 2000s that we needed ice sheet models that could have high resolution around the key areas at the margins of the ice sheet—the grounding line where the ice meets the ocean.
BISICLES was created through a collaboration between CPOM scientists here in the UK, particularly Tony Payne and Steph Cornford at the University of Bristol, and scientists at Lawrence Berkeley National Labs. The Lawrence Berkeley team, particularly Dan Martin, had already developed software and tools that allow different resolutions in the model at different points in space.”

Adaptive Mesh Refinement

“Adaptive mesh capability is key to the BISICLES ice sheet model—the capability to have different resolutions in the ice sheet at different points in space. This means we can model in detail the areas around the margins of Antarctica where we need that high resolution to resolve the complex processes occurring there.”

Adaptive Mesh Refinement (AMR) in BISICLES enables scientists to create a grid of the ice sheet. The program can identify complex or rapidly evolving areas of interest and focus on these areas in high resolution to examine the processes occurring there. BISICLES AMR capability means the simulation continually adapts the mesh based on changing conditions in order to maintain accuracy.

Applications of BISICLES

BISICLES has been used by scientists to model past ice sheets, create simulations of what is happening to the Earth’s ice sheets right now, and project how ice sheets may behave in the future subject to our changing environment and climate change.

Past Applications: BISICLES was used recently in a project called BRITICE to reconstruct the British and Irish ice sheets at the last Glacial Maximum and understand where the ice streams and glaciers were within that ice sheet, and how they formed the landscape we see today.
Present-Day Research: BISICLES is used by people all around the world looking at ice in the present, trying to understand how Antarctica is changing today and why it’s changing at the pace that it is—and the same for Greenland, studying some of the key ice streams and glaciers there.

Climate Modelling: Recently, BISICLES has been coupled to the UK’s Earth System Model (UKESM), so it’s now the ice sheet component in a wider global climate model. This is a really nice feature, allowing us to explore the feedbacks between ice sheets in Antarctica and Greenland on ocean circulation, atmospheric circulation, and the interactions that are going on. That’s a really new capability that BISICLES has by being part of this collaboration through CPOM.

UK Earth System Model (UKESM)

The UK Earth System Model is a partnership project across eight NERC centres and the Met Office, which develops and uses advanced Earth System models to support UK and international efforts to understand and tackle climate change, focusing on components including interactive land ice sheets. UKESM is the UK’s first earth system model based on the Met Office core global climate model, HADGEM3. UKESM provides robust projections of the Earth system response to anthropogenic climate forcing, incorporating all relevant processes and interactions.

“If we can accurately simulate what’s happening right now, we can then run our simulations into the future and try to learn about how our planet might change under a variety of different future scenarios.”

International Collaboration: ISMIP

BISICLES is also used in big international projects like ISMIP to project future changes to global sea levels.

“ISMIP is the Ice Sheet Model Intercomparison Project, a community effort bringing many ice sheet modellers together to produce a set of projections of the contribution of Antarctica and Greenland to future sea level rise, and also to further our understanding of processes at play and to better quantify the uncertainties for future projections of sea level rise.”

Looking to the Future

“BISICLES and other models like it provide us with an understanding of the key processes governing Antarctica and Greenland, and that better understanding is what allows us to produce more robust projections of future sea level rise.”

“We can’t predict the future – we don’t have a crystal ball—but we can look at all the different possibilities of the future and see what Antarctica might look like, depending on, for example, how much greenhouse gases are emitted by people over the coming decades.”