Scientists weighed the glaciers and found out they were so thin...

In December 2022, the United Nations General Assembly formally passed a resolution to establish March 21st of each year as World Glacier Day starting from 2025. Although the glacier festival will not begin until next year, a recent study in Science Advances still forces us to continue to pay attention to glaciers - 40% of Antarctic ice shelves have shrunk significantly in the past 25 years, showing the significant impact of human activities on the polar environment. Among them, 48 glaciers have lost more than 30% of their weight, and 7.5 trillion tons of meltwater have entered the sea, which may bring a series of chain reactions by affecting ocean circulation.

On a beautiful summer day, icebergs melt off the coast of Greenland (copyright image from the gallery, reprinting may cause copyright disputes)

01 How to know the “weight” of a glacier?

How do scientists know about changes in the area, thickness, and weight of glaciers?

There are many ways to calculate the area of ​​glaciers. In the past, we could draw a glacier distribution map and count the grids on a grid paper. Now, we can measure the area of ​​glaciers by using drones and satellite remote sensing images , and use computers to get specific area data.

The thickness of a glacier can be measured directly at the profile or by drilling . If there is a steep glacier profile, a measuring rope can be pulled vertically from the top to the bottom or the profile height can be directly measured with a rangefinder to obtain the thickness of the glacier at that point. Drilling is more complicated. It is necessary to ensure that the well is vertical and reaches the bottom of the glacier. The depth of the well is the thickness of the glacier.

This is the Rongbuk Glacier at the foot of Mount Everest ( Source: Xinhua News Agency )

It is also possible to obtain glacier thickness information more conveniently with the help of instruments such as gravimeters and ground-penetrating radars . For example, in 1968, scientists used gravimeters to observe ice thickness data of two sections of the Rongbuk Glacier at the foot of Mount Everest. In 2009, a more comprehensive thickness measurement of the East Rongbuk Glacier was carried out using ground-penetrating radars.

The black lines L1-L8 in the figure are the distribution of the ground penetrating radar thickness measurement lines of the East Rongbuk Glacier on Mount Everest, the red lines C1-C5 are the distribution of the glacier trough sections, and the blue lines are the main glacier lines. The end of the glacier is near C1, and ground penetrating radar wave velocity measurements were carried out at P1 and P2. [1]

The ground penetrating radar wave velocity measurement results at P1 and P2 clearly show the ice-rock interface. The speed corresponding to the bright spot, 0.13 m/ns, is the average wave velocity between the glacier surface and the ice-rock interface. Multiplying this by the time of 400 ns, we can get that the glacier thickness at P1 and P2 is about 52 m. [1]

These are radar measurements of L1-L8 of the Dongrongbuk Glacier, which clearly show the thickness of the ice layer and the subglacial topography. [1]

With the glacier thickness at multiple points, the average thickness and maximum thickness of the glacier as a whole can be calculated through computer models. The more points there are and the more evenly distributed they are, the more accurate the data will be . For example, glacier thickness measurement data show that the average thickness of the Dongrongbuk Glacier is about 190 meters and the maximum thickness is about 320 meters. With the area and average thickness, the volume data of the glacier can be obtained, and then multiplied by the average density of the glacier to obtain the weight data of the glacier. By comparing data from different periods, we can understand the changes in the weight of the glacier.

02 What are the impacts of meltwater entering the sea?

Glaciers store a large amount of land fresh water. If this fresh water enters the ocean in a short period of time, it will significantly reduce the salinity of the sea water, break the original balance of the sea water, and affect the ocean movement of ocean currents .

There are many reasons for the formation of ocean currents. Some are caused by prevailing winds such as trade winds and westerlies blowing over the sea surface. When the seawater starts to flow, some places have more seawater and some places have less seawater. The places with more water will flow to the places with less water. This is the compensation flow. Some are also affected by density differences. For example, the seawater density on both sides of the Strait of Gibraltar is different. The density on the Mediterranean side is higher and the density on the Atlantic side is lower. The surface seawater flows from the Atlantic into the Mediterranean, and the deep seawater flows from the Mediterranean into the Atlantic. This is the density current.

Schematic diagram of ocean currents ( copyright image from the gallery, reprinting may cause copyright disputes )

Different ocean currents carry hot or cold water to other seas, promoting heat exchange between different seas. Western Europe is affected by the North Atlantic warm current, which brings a lot of water vapor and heat. Although it is at a high latitude, the average temperature of the coldest month in winter is above zero degrees, forming a temperate marine climate that is mild and humid all year round.

When the density of seawater is reduced by meltwater, the original ocean current may be interrupted, greatly changing the climate, vegetation and other environments in the relevant areas. In the Quaternary geological history, due to climate warming, the melting of glaciers in North America entered the sea and changed the properties of seawater, causing the interruption of the North Atlantic warm current, and western Europe fell into a very cold and dry environment.

03 Can meltwater be stopped from entering the sea?

Since meltwater entering the sea will have such a huge impact on ocean currents, climate, vegetation, etc., can we humans directly stop the meltwater from entering the sea? The answer is no, because the global glaciers are huge. The meltwater volume of 48 of the many glaciers in Antarctica alone reached 7.5 trillion tons in 25 years. It is still impossible for humans to stop so much water from entering the sea at their current level.

Some people say that since glaciers are fresh water, wouldn't the melted fresh water be used by humans to solve the water shortage problem? The idea is beautiful, but it is very difficult to implement. The Antarctic continent is far away from other densely populated areas, and the melted water needs to be stored in huge containers and then transported to water-scarce areas, which is very costly.

The United Arab Emirates once planned to transport Antarctic icebergs to the Middle East to solve its own country's water shortage problem, but it finally gave up because it was unable to solve the problem of meltwater storage along the way and the huge cost due to the long distance.

A polar bear on an ice floe in the waters of Svalbard, northern Norway, in 2006 (Source: Xinhua News Agency)

04 Is there any other solution?

Are we just going to watch the melting of Antarctic glaciers and their huge impact on the global environment without being able to do anything about it? Not entirely. The fundamental problem of the melting of Antarctic glaciers is global warming, which is caused by greenhouse gas emissions represented by carbon dioxide. The person who tied the bell must untie it. Only by fundamentally solving the problem of excessive carbon dioxide emissions can this problem be avoided.

Comparison of the overall view of Fox Glacier in 2012 (left) and the aerial view of Fox Glacier on April 19, 2021 (right) (Source: Xinhua News Agency)

At present, many countries around the world have put forward their own dual carbon goals. China has pledged to peak its carbon emissions before 2030 and achieve carbon neutrality before 2060, that is, the amount of carbon dioxide emissions and absorption offset each other to achieve net zero carbon dioxide emissions.

In order to better achieve the dual carbon goals, countries have increased their carbon emissions and increased carbon absorption in multiple areas such as new energy and energy conservation and emission reduction. They have increased the use of low-carbon energy such as solar heating, wind power, and hydropower, reduced the proportion of fossil fuels that emit a large amount of carbon dioxide, and increased energy utilization. They have planted trees to increase the absorption of carbon dioxide by plants. When the net carbon dioxide emissions no longer continue to increase, the rate of global warming may be effectively alleviated, and the problem of Antarctic glacier retreat may be effectively controlled.

References:

【1】Zhang Tong, Xiao Cunde, Qin Xiang, et al. Thickness measurement and topographic characteristics analysis of the East Rongbuk Glacier on Mount Everest[J]. Journal of Glaciology and Geocryology, 2012, (5): 1059-1066.

Author: Yang Shuaibin, PhD in Quaternary Geology, Chinese Academy of Geological Sciences, Geography Teacher at Beijing No. 11 School

Reviewer: Dong Hanwen, Associate Researcher, Institute of Geology, Chinese Academy of Geological Sciences

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.