Research from the Cavendish Laboratory has found an unexpected large-area greening trend along the Pacific coast of northern Peru and Chile, with implications for regional environmental management and agriculture. A team from Cambridge's Cavendish Laboratory analyzed satellite data over the past 20 years to study how vegetation has changed along the Pacific coast of northern Peru and Chile. This region is known for its unique and delicate arid and semi-arid environments.
The analysis showed that some areas experienced a positive growth of vegetation, known as "greening", while others experienced a negative trend, known as "browning". It is not difficult to see that changes in vegetation are affected by factors such as agricultural and urban development or changes in land use patterns.
Discover significant greenery
But even more interestingly, the study, published in MDPI Remote Sensing, found that a large swath of the western slopes of the Andes is experiencing significant greening over the past 20 years. This section stretches about 2,000 kilometers from northern Peru to northern Chile and has seen a significant increase in vegetation over time. This greening trend changes with altitude, and different altitudes have different vegetation types.
A research team composed of mathematicians, geographers, biologists and earth scientists used satellite images from 2000 to 2020 to observe changes in the area's vegetation over time. They plotted 450 data points and developed a mathematical model to remove human-made variations (such as cloudy days) and seasonality, and used statistical analysis to ensure only areas with significant trends were analyzed.
An exaggerated three-dimensional model of the Peruvian Andes with green belts superimposed on it, with green areas representing increases in the enhanced vegetation index and dark areas representing higher relative greenness. Source: Hugo Lepage, Cavendish Laboratory.
"We spent three years putting together the methods and statistical models, working very carefully to make sure that something really large-scale was happening and wasn't just a fluke," said Hugo Lepage, a mathematician at the Cavendish Laboratory and lead author of the study.
Ground observations corroborate satellite data
To verify what they saw in the data, the researchers conducted multiple field trips, making observations on the ground to corroborate their numerical statements.
Eustace Barnes, a geographer in the Environmental Physics Group at the Cavendish Laboratory, explained: "We started with a very local area to study the impact of mining on local vegetation. To our surprise, the data showed that the area was greening, not browning. So we zoomed in and found that other areas were also greening on a large scale. When we went to look in the field, we observed similar trends."
In addition to empirical observations of the green belt itself, the researchers were struck by its surprising characteristics.
"First, as we look southward, the green belt gradually rises, from 170-780 meters in northern Peru to 2,600-4,300 meters in southern Peru. This is counterintuitive because we would expect surface temperatures to decrease both as we move southward and as we gain altitude," explains Barnes.
Even more surprising is that this vast green belt does not coincide with the climate zones identified by the Köppen-Geiger classification - a widely used vegetation-based empirical climate classification system - but closely matches the greening and browning trends in coastal deserts and Andean highlands.
"In fact, in northern Peru, the green belt is primarily located in a climate zone that corresponds to hot, arid deserts. As we scanned the green belt southward, it gradually rose higher, mostly in hot, arid savanna, and finally crossed into cold, arid savanna. This was inconsistent with what we expected based on the climate of these regions."
Implications and conclusions
The results of this study have far-reaching implications for environmental management and policy development in the region. While the exact causes or consequences of this greening are not yet known, any large change in vegetation (a 30-60% increase in the index) is bound to have an impact on ecosystems and the environment.
"The Pacific Slope provides water to two-thirds of Peru and much of Peru's food comes from here," said Barnes. "Such rapid changes in vegetation and water levels and ecosystems will inevitably have consequences for water resources and agricultural planning and management."