Amazon Nears “Hypertropical” Climate, Threatening Global Carbon Sink
The Amazon rainforest, a vital global ecosystem, is on the precipice of a significant climatic shift, potentially entering a “hypertropical” state unseen for tens of millions of years. This transformation, detailed in a recent study, paints a concerning picture of a hotter, drier, and more volatile environment that could lead to widespread tree mortality and severely compromise one of Earth’s most crucial carbon sinks.
Scientists are warning that without drastic reductions in greenhouse gas emissions, the Amazon could face up to 150 days of “hot drought” conditions annually by the year 2100. These are periods of extreme dryness exacerbated by intense heat, and alarmingly, they could occur even during months typically characterized by heavy rainfall, such as March, April, and May. Such conditions are virtually unheard of in the Amazon today.
“When these hot droughts occur, that’s the climate we associate with a hypertropical forest,” explained lead author Jeff Chambers, a professor at the University of California, Berkeley. “It’s beyond the boundary of what we consider a tropical forest now.”
Unraveling the Amazon’s Breaking Point
The groundbreaking study, spearheaded by researchers at the University of California, Berkeley, analyzed over three decades of environmental data. This included measurements of temperature, humidity, soil moisture, and light intensity collected from research plots situated north of Manaus in central Brazil.
Crucially, sensors embedded in tree trunks provided researchers with real-time insights into how these trees react to increasing heat and diminishing moisture. During recent El Niño-driven droughts, scientists identified two primary points of stress for the Amazonian flora.
Water Conservation and Carbon Uptake: When soil moisture levels plummeted to approximately one-third of their normal values, a significant number of trees began to close their leaf pores. This adaptive measure, aimed at conserving water, had a detrimental side effect: it severely hindered their ability to absorb carbon dioxide, the essential element they require for growth and tissue repair.
Sap Transport Disruption: Prolonged periods of intense heat led to the formation of bubbles within the trees’ sap. This phenomenon disrupted the crucial transport of water throughout the plant, a process researchers likened to an embolism – a sudden blockage in a blood vessel that can lead to a stroke in humans.
The study highlighted that fast-growing trees with low wood density were particularly susceptible to these stresses, exhibiting higher mortality rates compared to trees with denser wood.
“That implies that secondary forests might be more vulnerable,” Chambers noted, “because secondary forests have a larger fraction of these types of trees.” Secondary forests are those that have naturally regenerated after being damaged by human activities or natural events.
The researchers observed these consistent warning signs across multiple study sites and during various drought events. This uniformity suggests that the Amazon’s response to heat and dryness is predictable and follows a similar pattern throughout the region.
While annual tree mortality in the Amazon currently hovers just above 1 percent, the study’s projections indicate this figure could rise to approximately 1.55 percent by 2100. Although seemingly a minor increase, Chambers emphasized that even a half-percentage-point rise across the vast expanse of the Amazon represents a staggering loss of trees.
Understanding the “Hypertropical” Climate and Its Implications
The research team defines a “hypertropical” climate as one that is hotter than 99 percent of historical tropical climates and is characterized by significantly more frequent and intense droughts. Such a climate has no modern precedent. It is believed to have existed in tropical regions only during periods when Earth was considerably warmer, between 10 and 40 million years ago.
In stark contrast to the relatively stable temperatures and consistent rainfall that support the Amazon’s dense vegetation today, a hypertropical climate would usher in extreme heat, prolonged dry seasons, and the potential for severe storms.
The consequences of this climatic transition would extend far beyond the Amazon basin itself.
Carbon Sink Vulnerability: Tropical forests are unparalleled in their capacity to absorb atmospheric carbon dioxide. However, when subjected to stress, their carbon uptake diminishes significantly. In particularly dry years, the Amazon has already been observed to release more carbon than it absorbs. As global temperatures continue to climb, any reduction in the Amazon’s ability to store carbon could accelerate worldwide warming and potentially contribute to it. Recent years have seen severe fire seasons in some rainforests, fueled by heat and drought, leading to substantial carbon releases and ecosystem stress.
Global Ripple Effects: The changes occurring in the Amazon could have cascading impacts on other forest ecosystems. The study’s authors assert that rainforests in western Africa and Southeast Asia may face similar risks as global temperatures rise, with the severity depending on the pace and scale of emission reductions.
“It all depends on what we do,” Chambers concluded. “If we’re just going to emit greenhouse gases as much as we want, without any control, then we’re going to create this hypertropical climate sooner.”