Information gathered from the International Space Station indicates that a minor proportion of leaves within the global tropical rainforests are presently surpassing peak temperatures. Experts caution that this situation has the potential to escalate further.
A recent study has cautioned that climate change might gradually render the world's tropical rainforests unsuitable for photosynthesis, potentially leading to their collapse. The study utilized data acquired from the International Space Station (ISS) and revealed that a small yet expanding percentage of tree leaves within tropical forests are nearing the upper temperature threshold required for photosynthesis.
The critical average temperature at which the photosynthetic machinery in tropical trees begins to malfunction is 116 degrees Fahrenheit (46.7 degrees Celsius). Presently, only 0.01% of all leaves exceed this critical temperature each year. However, scientists are raising concerns that if air temperatures rise by 7.2 degrees Fahrenheit (4 degrees Celsius), it could drive trees in tropical forests beyond a tipping point, resulting in extensive mortality.
In case such a scenario unfolds, it could signify catastrophe for both Earth's climate systems and biodiversity, as highlighted by researchers in a study published on Wednesday (August 23) in the journal Nature.
"It's alarming to observe nonlinear patterns. For instance, you raise the air temperature by, let's say, 2 or 3 degrees Celsius [3.6 to 5.4 F], and the upper temperature limit of these leaves surges by 8 degrees Celsius [14.4 F]," explained Christopher Doughty, an associate professor of ecoinformatics at Northern Arizona University, during a press briefing on Monday (August 21). "Although only a small fraction of leaves are currently encountering this, our most informed estimate suggests that a temperature rise of 4 degrees Celsius could pose significant challenges for specific tropical forests."
Measuring the Temperature of a Rainforest
Tropical rainforests hold immense importance for our planet. Spanning over 3 billion acres (1.2 billion hectares), approximately 6% of Earth's total surface, they provide habitat for half of the global animal and plant species. These regions are crucial reservoirs of freshwater, with the Amazon Basin alone storing one-fifth of the world's fresh water. Notably, rainforests' photosynthesis contributes to 32% of the Earth's oxygen production and plays a pivotal role in stabilizing the global climate by absorbing billions of tons of carbon dioxide from the atmosphere annually.
To gain insights into the temperature conditions within the world's tropical forests, researchers utilized the Ecosystem Spaceborne Thermal Radiometer Experiment on the Space Station (ECOSTRESS) sensor located on the ISS.
They combined temperature data obtained from ECOSTRESS spanning from 2018 to 2020 with numerous ground measurements collected from infrared-sensing pyrgeometers positioned in rainforests across South America, Central Africa, and Southeast Asia.
Through the compilation of these findings, it was revealed that the average peak canopy temperature reached approximately 93.2°F (34°C), with a small percentage surpassing 104°F (40°C). Furthermore, each season, a mere 0.01% of leaves exceeded a critical temperature threshold that would likely result in the cessation of photosynthesis and their subsequent demise.
Although this percentage might seem minor, the researchers emphasized its potential for rapid escalation. Joshua B. Fisher, an associate professor of environmental science at Chapman University in California, noted, "While the number is small, it has significant implications — it won't just go from 0.01 to 0.02. The increase will be nonlinear, and it could accelerate much faster."
Laboratory experiments involving leaves subjected to warming of 2°C, 3°C, and 4°C were conducted. These revealed that temperatures around certain leaves peaked considerably higher than the surrounding air temperature — by up to 14.4°F (8°C).
Upon integrating these peak temperatures into a mathematical model, the researchers determined that a mean air temperature increase of 3.9°C (7°F) around the leaves led to the closure of water-carrying stomata by the trees, primarily affecting those leaves exposed to the heightened heat. This response resulted in their demise and set off a chain reaction, elevating temperatures around the remaining leaves and potentially leading to their death as well. This sequence could subsequently extend to branches and entire trees.
"Once you have 10% of the leaves dying, it will impact the entire branch's temperature regulation since a crucial part of that branch can no longer contribute to cooling the wider section of the branch. This principle applies across the entire forest as well, following the death of a tree," explained Doughty.
Despite their discoveries, the researchers hold an optimistic viewpoint, suggesting that humanity still possesses a window of opportunity to reduce emissions and avert potential tipping points within tropical forests.
"This provides a glimpse into a potential tipping point. It doesn't imply that tropical forests will immediately transform into savannas overnight," Fisher pointed out. "If you draw a parallel with human health, you'd want to be aware of any illnesses or potential cancer so that you can address it before it becomes uncontrollable."
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