Rainforests in Australia’s tropical north become first to shift regime from carbon sink to source

Rainforests in Australia’s Tropical North Become First to Shift from Carbon Sink to Carbon Source

Analysis:

1. In a first-of-its-kind finding globally, rainforests in Australia’s tropical north — particularly in Queensland — have shifted from acting as carbon sinks to carbon sources, releasing more carbon dioxide than they absorb.
2. The study, conducted by scientists from the Australian National University (ANU) and published in the journal Nature, found that the transformation began around the year 2000, driven by climate change, rising temperatures, drought, and cyclones.
3. Between 1971 and 2000, the forests acted as carbon sinks, storing around 0.62 Mg C ha⁻¹ yr⁻¹(megagrams of carbon per hectare per year).
   • However, from 2000 onwards, they became carbon sources, losing -0.93 Mg C ha⁻¹ yr⁻¹.
   • The decline in sink capacity was observed at a rate of 0.041 Mg C ha⁻¹ yr⁻¹.
4. Researchers found that increased tree mortality — driven by climate extremes such as higher temperatures, vapour pressure deficits (VPD), and maximum climatological water deficits (MCWD) — was the primary factor behind the shift.
5. Despite rising CO₂ concentrations, which should theoretically enhance photosynthesis and tree growth, the mortality rate exceeded biomass gains, leading to net carbon release.
6. Cyclonesplayed a significant but secondary role in the shift. They were found to:
   • Increase tree mortality rates by 19% above background levels,
   • Weaken the carbon sink capacity for up to six years post-event.
7. Lead author Hannah Carle (Western Sydney University) emphasized that this marks a critical warning signfor global climate targets:

   “The ability of woody biomass to act as a carbon sink is increasingly at risk. The losses from tree mortality are not being balanced by new growth.”

8. The study warns that cyclone intensity and frequency are likely to increase with climate change, potentially expanding this phenomenon to other tropical forest regions worldwide.

Static & Conceptual Insight:

1. Carbon Sink: A natural system (like forests, oceans, or soil) that absorbs more carbon dioxide than it emits.
2. Carbon Source: A system that emits more carbon dioxide than it absorbs.
3. VPD (Vapour Pressure Deficit): Indicator of atmospheric dryness — higher VPD stresses plants, increasing mortality.
4. MCWD (Maximum Climatological Water Deficit): Reflects long-term drought stress.
5. Location:
   • Region: Northeastern Australia
   • Major sites: Barron Gorge National Park, near Cairns, Far North Queensland
6. Global Significance:
   • The shift challenges the assumption that tropical forests will continue offsetting CO₂ emissions.
   • Has direct implications for global carbon budgets and maintaining temperature rise below 2°C, as per the Paris Agreement.
     
     Updated -  17 Oct 2025, 4:34 pm | Down to earth