Multifaceted characteristics of arctic and boreal evapotranspiration in a warming world from multi-source remote sensing

Abstract

Total evapotranspiration (ET) trends remain highly consistent across FLUXCOM-X-BASE and GLEAM v4.2a, indicating a robust growing-season ET change in northern high latitudes. However, the mechanisms behind this change are less consistent than the total flux suggests. This thesis examines whether vegetation greening explains ET changes, whether ET components compensate for one another, and whether dry-wet ecosystem classifications represent process-level ET controls.
Growing-season ET north of 45°N from 2001 to 2020 was analyzed using FLUXCOM-X-BASE and GLEAM v4.2a, together with MODIS leaf area index (LAI), land surface temperature (LST), albedo, and the GLWD v2.0 dry-wet classification. The analysis focused on total ET trends, component partitioning, nonlinear temperature responses, and pixel-wise driver attribution.
Although total ET trends are broadly consistent between the two products, their inferred mechanisms differ substantially. Dry ecosystems show a near-compensatory substitution among ET components, whereas wet ecosystems show weak or nearly absent component changes. The direction of this substitution is broadly consistent with vegetation greening, but the relationship between pixel-level Ec/ET trends and LAI trends is very weak, with an R² of approximately 0.02. This suggests that greening alone cannot explain the magnitude of ET component changes, and that non-vegetation processes are also important.
ET responds nonlinearly to LST anomalies. A cold-end breakpoint near -9°C likely reflects constraints from snow cover and vegetation dormancy rather than water limitation. When stratified by latitude, an inverted-U warm-end response appears in the 45-55°N band for both dry and wet pixels, suggesting that the transition from energy limitation to water limitation follows a latitudinal gradient rather than the GLWD-defined dry-wet split.
Pixel-wise partial correlations further show that dry and wet ecosystems within the same latitude band have similar driver structures, with the largest dry-wet difference reaching only 0.08 absolute units for any single driver. Latitude separates ET controls more clearly: LST and soil moisture correlations strengthen toward higher latitudes, while LAI weakens slightly. Finally, driver attribution differs strongly between products. The LAI-dominant pixel share drops from 75% and 65% in dry and wet pixels under FLUXCOM-X-BASE to 34% and 29% under GLEAM v4.2a, whereas LST correlations remain relatively stable. This divergence likely reflects predictor circularity in FLUXCOM-X-BASE, whose machine-learning framework uses MODIS-derived variables that overlap with the driver set. Overall, total ET trends are robust across products, but mechanistic attribution is highly product-dependent. Future large-scale ET studies should therefore validate findings across multiple datasets and critically examine internal model assumptions before drawing causal conclusions.

Popular Abstract

The far north is warming faster than most other parts of the Earth. As temperatures rise, many Arctic and boreal landscapes are becoming greener, with shrubs, grasses and trees growing more actively or expanding into new areas. This greening does not only change what the land looks like. It can also change how water and heat move between the land and the atmosphere.
A key process in this exchange is evapotranspiration. This means all the water that leaves the land surface and returns to the air: water evaporating from soil, water evaporating from wet leaves, and water released by plants through their leaves. Evapotranspiration matters because it uses heat energy. When more energy goes into evaporating water, less energy is left to warm the air directly. In this way, changes in evapotranspiration can affect whether greener northern landscapes help cool the surface during summer or contribute to further warming.
This thesis examined growing-season evapotranspiration across northern land areas above 45°N from 2001 to 2020. Rather than looking only at the total amount of water leaving the land, it asked a more detailed question: has greening changed the pathways through which water leaves the surface? It also compared dry and wet ecosystems, tested how evapotranspiration responds to unusually warm or cold months, and examined which environmental factors are most closely linked to evapotranspiration changes.
The results show that total evapotranspiration has generally increased, and this pattern is similar in two independent evapotranspiration datasets. However, the internal changes behind this increase are more complex. In dry ecosystems, the share of evapotranspiration coming from plant transpiration increased, while the share coming from soil evaporation decreased by almost the same amount. This means that greening did not simply add more water loss to the system. Instead, it changed the route by which water left the land: less directly from bare soil, and more through plants. Wet ecosystems did not show the same clear shift, probably because many of them already had enough water and relatively high plant water use.
The relationship between temperature and evapotranspiration also depended strongly on latitude. In the coldest northern areas, warmer months generally increased evapotranspiration because they reduced cold limitations and allowed vegetation and evaporation to become more active. Farther south, especially between 45°N and 55°N, very warm conditions sometimes caused evapotranspiration to level off or decline. This suggests that in these warmer parts of the northern region, water supply can become limiting when temperature and atmospheric demand become too high.
Another important finding is that the simple division between “dry” and “wet” ecosystems did not fully explain how evapotranspiration behaves. Latitude and temperature background often separated the patterns more clearly. The analysis also showed that different evapotranspiration datasets can agree on total trends while disagreeing on which driver appears most important. This means that conclusions about the causes of evapotranspiration change should not rely on only one data product.
Overall, this work shows that a greener north does not automatically mean a simple increase in water loss or cooling. The effect depends on whether water leaves through plants or soil, where the ecosystem is located, how warm it already is, and how the data product represents land-surface processes. Understanding these details is important for predicting how northern ecosystems will feed back to future climate warming.

• author: Yuan, Yaxian
• supervisor: Wenxin Zhang
• organization: Department of Earth and Environmental Sciences (MGeo)
• course: NGEM21 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Evapotranspiration, Vegetation greening, Arctic, Remote sensing, Climate change, Global warming.
• publication/series: Student thesis series INES
• report number: 785
• language: English
• id: 9239373
• date added to LUP: 2026-06-16 13:58:39
• date last changed: 2026-06-16 13:58:39