Cryosphere: an oracle of past, present, and future climate 

The cryosphere, where water is in a frozen state, is a critical component of the climate system. The cryosphere consists of glaciers, ice sheets, icebergs, snow, permafrost, and land and sea ice. Each of these entities works differently and has varying climatic implications.  

Ongoing warming has brought unpredictable changes in the cryosphere. While the Arctic and Himalayas are facing rapid melting, Antarctica is gaining ice mass. These changes impact Earth’s albedo and alter the water cycle and freshwater budget. More importantly, cryospheric melting results in sea level rise which has far-reaching consequences on human civilization and other forms of life.  

India's monsoon, agriculture and economy are closely entangled with one of such cryospheric ecosystems- the Himalayas, which is known as the third pole. Recently, Lavkush Kumar Patel and team from the NCPOR studied major glaciers in the western Himalayas covering the Chandra, Bhaga, Miyar and Parvati sub-basins in the Indus basin. They used data from satellites, reanalysis and models from 1971 to 2018 to study the dynamics of the glaciers. They also used data from ‘Himansh’- NCPOR’s high altitude autonomous weather station at the Chandra basin to validate the analysis. 

The team found a ten per cent decrease in the glacier area over the five decades. The maximum area loss was in the Parvati basin. However, the Bhaga basin lost the maximum fraction of its glaciers which was around 15 per cent. They observed a faster loss of area in steeper, smaller, eastward flowing glaciers at higher altitudes. The researchers related this decline to a combined effect of increasing air temperature and decreasing snow precipitation. This combination caused an early onset of glacier ice melting and elongated the melting period which resulted in a decrease in the glacier. The team also observed an increase in liquid precipitation further contributing to the decline. 

Over the five decades, the researchers found the number of glaciers increased by 14 in the western Himalayas. However, this increase was a result of melting and retreat that broke larger glaciers into smaller tributary glaciers. Hence, in this case, an increase in the number of glaciers is definitely not good.  

*Schematic representation of (A) change in western Himalayan glaciers over five decades (Lavkush Patel et al., 2021), (B) surface mass balance in ice rises and ice shelf in Dronning Maud Land, Antarctica (Bhanu Pratap et al., 2021), and (C) reconstruction of sea ice using ice cores from Dronning Maud Land (Tariq Ejaz et al., 2021).  

Changes in glaciers can also be quantified by assessing the surface mass balance. This mass balance is the seasonal difference between the accumulation and melting of snow and ice. However, accumulation especially on ice shelves- the connecting portion of glaciers between land and ocean, doesn’t happen uniformly. Elevated topography results in the formation of ice rises- the dome-shaped structures on ice shelves. Due to its proximity to the oceans and large accumulation, ice rises are important to understand past climatic variations.  

A team led by Bhanu Pratap, NCPOR studied the surface mass balance of the Leningradkollen and Djupranen ice rises of the Nivilisen ice shelf in the Dronning Maud Land, Antarctica. The team used ice-penetrating radar to visualise the stratigraphy of firn- the compressed and crystallised snow, in the top 35 metres across the ice shelf from 1986 to 2017. In the ice rises, they identified six isochrones connecting events that occurred at the same time or with the same time difference. The researchers dated the isochrones using ice cores retrieved from the ice rises. For the periods demarcated by the isochrones, they studied the changes in the surface mass balance.   

Overall, the team found a similar mass balance during the three decades. However, there was a strong east-west gradient between the ice rises. They found a low surface mass balance in the inlet of Leningradkollen and an increase towards the Djupranen. The researchers attributed this strong gradient to the regional orographic differences. While comparing the radar-derived mass balance data with the model simulations, they found discrepancies. 

Models are continuously trained and validated to produce accurate processes even in the absence of observations. However, existing regional climate models do not consider small scale features like ice rises to lead to biases while estimating the surface mass balance, say the researchers. So, the team emphasizes the need of improving the models by incorporating ice rise dynamics. 

Ice cores are being used to understand past environmental processes on a high-resolution interannual timescale too. Recently, Tariq Ejaz and team from the NCPOR deciphered sea ice conditions in the western Indian Ocean sector of the Southern Ocean surrounding Antarctica. They used an ice core retrieved from the coastal Dronning Maud Land dated 1809-2013. The team measured the oxygen isotopic ratio between the heavier oxygen-18 and the lighter oxygen-16 and reconstructed sea ice variability at an interannual resolution.

The oxygen isotopic ratio is shaped by many climatic factors where the major contribution comes from temperature and sea ice. The researchers found that for the previous inland cores from the Dronning Maud Land, temperature shaped the isotopic variability. However, the coastal ice core was dominantly influenced by sea ice variability. 

The team derived a mathematical relation and used it to reconstruct the past sea ice by correlating the isotopic data from the coastal core to the sea ice reanalysis data. Earlier studies reported an inverse relationship between isotopic ratio and sea ice. This scenario holds when there is more ice in surrounding oceans and hence an isotopically depleted moisture reaches Antarctica from far away from open oceans. However, the team found a positive relationship between the isotopic ratio and the sea ice, which the researchers explained using the westerlies associated with the Southern Annular Mode.

The Southern Annular Mode is the pressure difference between the southern mid-latitudes and Antarctica and its positive phase shifts the westerlies poleward making them stronger. The team say that these westerlies pushed the sea ice from the adjacent Weddell Sea towards the east to the western Indian Ocean sector. As a result, a more exposed Weddell Sea became the source of isotopically heavy moisture for the east Dronning Maud Land. 

Over the two centuries, the team found a declining trend of sea ice from 1830 to 1884 and a moderate increase from 1927 to 1993. In recent decades, they checked the sea ice variability from satellite data and found a dramatic increase from 1994 to 2014. For 2015-16, they found a sharp decrease in the sea ice, which they related to El-Nino, a warm climate mode in the Pacific Ocean. The researchers comprehend that the degree of sea ice variability in the western Indian Ocean sector of the Southern Ocean is determined by the combination of phases of the Southern Annular Mode and distant teleconnections with Pacific oscillations. 

Being highly sensitive to changes, the cryosphere is a vital indicator of the past and future climate. Above studies by the NCPOR researchers highlight some of the key features and processes in different realms of the cryosphere and emphasise the need for more observations, paleoclimatic reconstructions and model improvements.

*Extra hand: Yuga Ghotge, research scholar