India is a vast country with incredibly diverse landscapes and rapid development. Over the past few decades, the nation has undergone significant changes in how its land is utilised, from the expansion of cities to shifts in agricultural practices. These changes are having a profound and sometimes surprising impact on temperatures nationwide. While we often hear about global warming caused by greenhouse gases, researchers are also studying how local changes to the land, such as building cities, deforestation, or altering farming practices, significantly impact regional and local climates.
In a new study from Dimoria College (Autonomous), Assam, and the Indian Institute of Technology (IIT) Bhubaneswar, researchers investigated how these land changes are affecting surface air temperature (SAT) in India. They used a variety of data collected between 2001 and 2015, including temperature readings from weather stations and satellite data, to examine how Land Use Land Cover (LULC) influences air temperature.
India hosts six major climatic subtypes, ranging from arid desert in the west, alpine tundra and glaciers in the north, and humid tropical regions supporting rainforests in the southwest and the island territories. |
The challenge is separating the effect of local land changes from the bigger picture of global warming that affects the whole planet. To achieve this, they employed a technique known as Observation Minus Reanalysis (OMR). They compared the actual temperature measurements with a computer model (reanalysis) that estimates temperatures but doesn't account for local land surface details. The idea is that the model captures the large-scale global warming trend while the actual observations capture both global warming and the local land effects. By subtracting the model's temperature trend from the observed temperature trend, the researchers could isolate the temperature change likely caused by changes in land use alone.
They found that the results varied across India. While the overall observed temperature across India showed a slight warming trend, the OMR analysis revealed that LULC changes were causing very different effects in different regions. In Southern, Eastern, and Western India, land use changes were contributing to warming. Specifically, in a key area studied in the South, land changes contributed approximately 0.54°C to the temperature rise. In a key area in the East, they contributed approximately 0.44°C.
But in Northern, Central, and North-Western India, the opposite was happening: land use changes were contributing to cooling. In a specific area of the North-West region, the cooling effect from land changes was quite significant, at around -1.5°C. This means that in some parts of India, local land changes are making warming worse, while in others, they might be counteracting some of the global warming effects.
To determine why this was happening, the researchers examined closely the types of land changes that occurred in these different regions. They used satellite data from 2004 and 2015 to map changes in land cover, such as the area covered by cities, farms, or forests. In the Eastern region, where warming was observed, there was a significant increase in Built-Up areas (cities and buildings), a jump of about 780%.
They also saw increases in farmland and forests but a significant decrease in unused fallow land. This rapid urbanisation is a major driver of warming, likely because buildings and pavement absorb and trap more heat than natural surfaces. In the Southern region, also experiencing warming, they found a significant increase in Rabi crops (grown in winter) and a decrease in Kharif crops (grown in summer). This shift in farming practices, known as crop diversification, was linked to the warming trend.
In the North-Western region, the land change data showed increases in Built-Up areas, Fallow land, and forests, along with slight decreases in Kharif and Rabi crops. The researchers also examined other data that provide clues about the land surface. They examined the Normalised Difference Vegetation Index (NDVI), a measure of the greenness and health of vegetation. They found a significant increase in NDVI (40-60%) in the North-Western region, indicating more vegetation.
They also observed a significant increase (80-90%) in evapotranspiration (ET), which is the process by which water evaporates from the soil and plants release water vapour. Both increased vegetation and increased evapotranspiration tend to cool the air. The study also noted that North-Western India has a high concentration of irrigated farmland, meaning water is actively supplied to crops. This intensive irrigation supports more vegetation and increases evapotranspiration, which helps explain the cooling effect observed in this region.
The researchers, however, acknowledge some limitations. They couldn't fully explain all the reasons for the cooling in the North-Western region and suggested that more detailed computer modelling is needed to isolate all the factors. They also point out that the atmosphere is complex, with many interacting factors, making it hard to pinpoint the exact impact of LULC alone. Finally, using data from different sources with varying levels of detail and quality can introduce some uncertainty. To improve this, they suggest future research should use high-resolution land data and more advanced computer models that can simulate different land-use scenarios across all of India.
Understanding how local land changes affect temperature is crucial for planning for the future. If we know that urbanisation causes warming, city planners can think about incorporating more green spaces or using different building materials to help cool things down. If we know that certain farming practices or irrigation methods can lead to cooling, this information can be used to inform agricultural policies and practices. By better understanding these complex interactions between the land and the air, researchers can provide more accurate predictions, helping policymakers make informed decisions to manage land and adapt to a changing climate.
This research article was written with the help of generative AI and edited by an editor at Research Matters.