Have you ever looked at a landscape and thought to yourself… How did this happen?! … Why is this hill here? … Why do these rocks look like this? … What does it all mean?
My job is to look for clues in the landscape that indicate the former presence of glaciers. Glaciers are masses of ice that flow downhill from mountain tops, where it is cold enough for snow to remain on the ground all year round. Rocks fall on the surface of glaciers and are transported to the glacier margins where they accumulate in elongated, sharp-crested ridges (as in the photo). These ridges are called moraines, and they can remain in the landscape long after the ice melts away. When I find moraines in a location where no glaciers currently exist, I ask myself these questions: (1) Why did a glacier used to be here? (2) When was it here? (3) Why is it no longer here?
I am here at a glaciology summer school in Alaska, USA, to learn about what causes glaciers to grow and shrink. Temperature is a key control as it determines how much precipitation falls as snow rather than rain in winter, as well as how much snow and ice melts in summer. When temperatures remain stable for a many years, glaciers eventually reach an equilibrium state, where input of snow equals output by melt, and the glacier length remains stable. If temperatures increase, then less snow may fall and/or more melt may occur, causing the glacier to retreat. If temperatures decrease, then more snow may fall and/or less melt may occur, causing the glacier to advance. As moraines represent former glacier lengths, they contain important information about past climate change.
My research focuses on two volcanoes in central North Island, New Zealand – a long, long way from Alaska! I have multiple moraines in several catchments on these volcanoes, which document former glacier length changes. I have dated these moraines using a technique known as “cosmogenic surface exposure dating.” This technique measures the accumulation of rare elements, produced by exposure to high-energy particles that come from outside our solar system and reach the Earth surface. The moraines on these volcanoes range in age from 200 to 60,000 years old. Now I am using a computer model that simulates the growth of glaciers under different climatic conditions. I input different temperatures to try to recreate the former glacier lengths, as indicated by the moraines I have found. Putting the moraine ages and model results together, I end up with estimates of how much colder it must have been in New Zealand at a certain point in time.
I know what you’re thinking … you’re thinking “who cares how cold it was thousands of years ago?” Well, that’s a great question. These results are important for several reasons. For example, they provide an important test for computer models that try to predict future climate change. If these models can recreate past changes, then we have increased confidence in their predictions for the future. You can think of this as cleaning the climatic crystal ball, making it easier to look into the future and see what Earth’s climate may be like for your children and grandchildren.
- Shaun, Victoria University of Wellington, New Zealand