Vegetation Amplifying Urban Heat? "Greening = Cool" Comes with Conditions: Why Plants Increase Heat in Arid Cities

"Planting Trees to Cool Down" is Not a "Universally Applicable Solution"

In cities experiencing extreme heat, greening has long been discussed as a standard heat mitigation strategy. The shade created by street trees and the cooling effect from evapotranspiration (evaporation of water from leaves, which absorbs heat) are indeed effective in many cities. However, a study introduced by Phys.org on January 5, 2026, adds a strong caveat of "dryness" to this conventional wisdom. In dry cities, greenery does not necessarily lead to cooling and may, in some cases, result in "net warming." Phys.org

What the Study Compared: Estimating "100% Vegetation vs. 100% Artificial Surfaces" in 761 Cities

The research team targeted 761 cities across 105 countries worldwide, combining high-resolution land cover data with satellite-derived land surface temperature (LST), and used machine learning to estimate the temperature difference (∆T) between scenarios where the area was entirely vegetation (trees, grasslands, farmland) and entirely artificial surfaces (buildings, pavements). The definition is simple: ∆T = T_veg − T_built-up. A negative value indicates cooling, while a positive value indicates warming. PMC

Note: LST refers to "surface heat" and may not align with pedestrian perceptions or air temperature (2m temperature). The researchers caution against directly linking the results to "perceived" temperatures, as urban shapes and shade configurations average out microclimates. PMC

Core Conclusion: Grass Cools in 78% of Cases but Warms in Dry Cities

Overall, "green cools" is the majority view. Grasslands were cooler than artificial surfaces in 78% of cases, and trees in 98%. Furthermore, the average cooling strength was greater for trees (average ∆T −3.71°C) than for grasslands (−1.44°C) or farmland (−1.86°C). PMC

However, the issue arises with the "reversal" in arid regions. In dry cities, where annual precipitation falls below an empirical threshold of about 1000mm, up to 22% of cities had grasslands and farmlands warmer than artificial surfaces. Even trees leaned towards warming in 13 cities in dry regions (about 2% of the total). PMC

A point often overlooked is that "greening" is not just about "trees." Urban policies tend to use "green coverage rate" as a blanket indicator, but the behavior of grasslands, farmlands, and trees differs, and under dry conditions, these differences become decisive.

Why the Reverse Effect Occurs: Weak Evapotranspiration Leads to "Albedo Loss"

The main player in cooling is evapotranspiration. However, in dry cities, water is scarce, and evapotranspiration (the ability to dissipate heat as latent heat) tends to weaken. As a result, changes in albedo (reflectivity) and heat storage become more prominent.

According to the Phys.org article, in dry cities, the "low albedo (less reflection)" and "changes in heat storage" of vegetation surpass the limited evapotranspiration, resulting in net radiative warming. Phys.org

The study also concludes that in dry areas, "cooling by evapotranspiration is outweighed by radiative warming due to albedo and changes in heat storage." PMC

In essence,without water, the "evaporative cooling engine" doesn't run. In such a state, increasing surfaces that absorb sunlight (i.e., less reflective) can create greenery hotter than concrete. Although counterintuitive, it makes sense in terms of heat balance.

Another Warning: Green Cooling "Stalls" During Extreme Heat Periods

Another critical point is that "the effectiveness drops during peak heat waves." The study defines extremely hot summers as "months where LST exceeds the 85th percentile of past summers" and examined cooling performance during this period. It found that trees failed to cool in 25% of cities, while grasslands and farmlands failed in 71% and 82% of cities, respectively. Phys.org

The study also explains that under extreme heat, trees are more resilient (maintaining evapotranspiration and turbulent exchange due to deep roots, leaf mass, and roughness), while grass and crops are vulnerable to water shortages due to shallow roots, leading to a drop in evapotranspiration. PMC

These results change the evaluation axis of policies. It's no longer sufficient to "lower the average of normal years"; the focus is now on what works during the life-threatening "peak days to weeks."

So, what to do? Reconfigure Greening to be "Climate Adaptive"

The conclusion is not that "greening is meaningless." On the contrary, trees show strong cooling in many cities and offer co-benefits (biodiversity, stormwater management, well-being, etc.). PMC

However, in dry cities, the "method" becomes critically important.

• Design Dry Cities with "Water"
  Water is needed if evapotranspiration is expected. Greening without the premise of "water design," such as efficient irrigation, native drought-resistant species, soil improvement, and rainwater storage, may not only be ineffective but also pose a risk of adverse effects. PMC

• Focus on "Canopy Placement" Over Grassland Expansion
  The study results suggest that grasslands and farmlands are more likely to be disadvantaged under dry conditions, while trees are relatively stronger. The idea of using canopies in conjunction with street design to cast shade where "people are" becomes effective. PMC

• Combine with High-Albedo Materials and Shade Structures
  The Phys.org article also suggests prioritizing water-efficient species and using complementary cooling strategies in dry cities. Instead of relying solely on greenery, combining methods to increase reflectivity, such as cool roofs, shades, and wind paths, is practical. Phys.org

SNS Reactions (As Far As Confirmed)

Since this is shortly after publication, there doesn't seem to be a large volume of posts to cover extensive reactions. On the Phys.org article page, it shows "43 shares" and "1 comment" at the time of retrieval. Phys.org

On the other hand, dissemination via official accounts can be confirmed. For example, Phys.org's LinkedIn post summarizes key points such as "Grasslands and farmlands can become warmer than buildings in dry cities (annual precipitation below 1000mm)" and "Even trees can fail to cool during extreme heat," with reactions (Likes) attached (3 at the time of retrieval). LinkedIn

Additionally, Science X / Phys.org's Threads post (headline and summary presentation) can also be confirmed in search results. Threads

Discussions likely to occur on social media generally split into two directions:

1. "Correcting the Course from 'Greening is Justice'": "Policies on lawns and green coverage rates that ignore regional water conditions are dangerous."

2. Realistic Countermeasures: "Dry cities should simultaneously implement cool roofs, shades, and other non-green measures."
   ——However, since the volume and trends of individual general user posts can vary greatly over time, this article is limited to what was "confirmed." Phys.org
   
   

References (Main Sources)

• Phys.org: “Vegetation might exacerbate urban heat island effect in very dry cities” (2026-01-05). Phys.org

• Guo, Z. et al., Science Advances (2026): “Global urban vegetation exhibits divergent thermal effects: From cooling to warming as aridity increases.” (PMC Full Text) PMC