Bromeliads on tree trunks in the Luquillo rainforest, Puerto Rico. Credit: Wikimedia Commons.

Since the mid-1970s, arthropod biomass in Puerto Rico’s tropical forests has declined by a factor of 60. Arthropods include insects, but also other invertebrates such as millipedes and sowbugs.

“Our results suggest that the effects of climate warming in tropical forests may be even greater than anticipated,” said Brad Lister lead author of the study and a faculty member in the Department of Biological Sciences at Rensselaer Polytechnic Institute. “The insect populations in the Luquillo forest are crashing, and once that begins the animals that eat the insects have insufficient food, which results in decreased reproduction and survivorship and consequent declines in abundance.”

Not enough climate action

Under the Paris Agreement, more than 190 nations pledged to lower their greenhouse gas emissions in order to avoid 2°C of warming compared to mid-19th-century levels. Although the increase might sound mild, the United Nations Intergovernmental Panel on Climate Change warns of severe consequences. To get an idea, even the difference between 1.5°C and 2°C of warming is to produce a global sea level rise of 10 cm, which would be devastating for many coastal areas. Furthermore, the likelihood of an iceless Arctic Ocean in the summer also increase 10 times from 1.5°C to 2°C. Most strikingly, coral reefs would decline by 70-90% with global warming of 1.5°C, whereas virtually all (> 99%) would be lost with 2°C, the IPCC authors of the Special Report on Global Warming of 1.5°C concluded recently.

The world is already 1°C warmer than pre-industrial levels, with devastating hurricanes in the US, record droughts in Cape Town and forest fires in the Arctic — so if we want to act, we should to so as urgently as possible.

But global warming is not uniform, with some parts of the world far warmers than others. For instance, the Arctic is warming faster than any other region on Earth and rapidly becoming a warmer, wetter, and more variable environment. Over the past 50 years, the Arctic’s temperature has risen at a rate more than twice the global average.

Puerto Rico’s tropical forests are also highly vulnerable, becoming a full 2°C warmer than they were only 40 years ago. In order to assess the effects of such a dramatic temperature elevation, researchers at the Rensselaer Polytechnic Institute studied data on arthropod abundance collected between 1976 and 2013 at three mid-elevation habitats in Puerto Rico’s protected Luquillo rainforest.

Based on arthropod samples collected in sticky traps on the ground and in the forest canopy, the biomass catch rates fell up to 60-fold between 1976 and 2013. Biomass collected at ground-level by sweep netting also declined by as much as 8-fold over the same period. Cold-blooded creatures living in tropical regions are particularly vulnerable to climate change because they’ve adapted to stable year-round temperatures.

Arthropods are the bottom of the food chain for many ecosystems. So it was no surprise to see that their decline simultaneously coincided with Luquillo’s insectivorous lizards, frogs, and birds.

Scientists estimate that planet’s tropical forests harbor two-thirds of all species. Given the dramatic collapse of Puerto Rico’s arthropods under just 2°C warming, the study published in the Proceedings of the National Academy of Sciences highlights just how vulnerable tropical ecosystems can be.

“Limiting global warming to 1.5°C compared with 2°C would reduce challenging impacts on ecosystems, human health and well-being, making it easier to achieve the United Nations Sustainable Development Goals,” said Priyardarshi Shukla, Co-Chair of IPCC Working Group III.

It is still possible to keep global warming under 1.5°C. However, doing so will require an unprecedented change from behalf of all stakeholders — and all of us. Another IPCC report released last week concluded that the goals of the Paris Agreement, while a major step in the right direction, are not ambitious enough — according to current projections, we’ll reach 1.5C by 2030.