Zooplankton makes carbon cycle models less certain

June 16 () –

Zooplankton, small animals near the bottom of the marine food chain, probably is the biggest source of uncertainty in the way we model the carbon cycle in the ocean.

Get your impact on the cycle correctly could add an additional 2 billion tons to the assumptions of current models of annual carbon uptake by the ocean. That’s more carbon than the entire global transportation sector emits, reveals new CSIRO researchAustralia’s national investigative agency.

Approximately 10 billion tons of carbon are released into the atmosphere each year. But the ocean quickly absorbs about 3 billion tons of these emissions, leaving our climate cooler and more hospitable.

However, the size of the ocean carbon sink is known to have changed in the past, and even small changes can cause large changes in the temperature of the atmosphere. Thus, we understand that the ocean acts as a thermostat for our climate. But, what does the dial control?

Extensive geological evidence suggests that microscopic marine life may be under control. Phytoplankton carry out photosynthesis and consumes as much CO2 as all land plants.

When phytoplankton die, they sink and trap much of their carbon in the deep ocean. It can stay there for centuries or millennia, safely locked away from contact with the atmosphere.

Any change in the force of this biological carbon pump will be felt in the atmosphere and will change our climate. Some have even proposed improving this biological pump by artificially fertilizing the ocean with iron to stimulate phytoplankton.

It is possible that this could sequester up to an additional 20% of our annual CO2 emissions.

Despite its importance to global climate and food production, there are large gaps in our understanding of how the marine carbon cycle is expected to change. Most Earth system models differ on how the major components of the cycle will respond to a changing climate. The models simply cannot agree on what will happen.

To diagnose what might be going wrong, the new study -published in Communications earth & environment-, compared the marine carbon cycle across 11 IPCC Earth System models. He found that the biggest source of uncertainty is how quickly zooplankton consume their phytoplankton prey, known as grazing pressure.

The models differ greatly in their assumptions about this pastoral pressure. Even if the zooplankton were exposed to the exact same amount of phytoplankton, the highest assumed grazing rate would be nearly 100 times faster than the slowest rate.

This is because some models effectively assume that the ocean is completely filled with slowly grazing shrimp. Others assume that it is exclusively filled with microscopic ciliates, but that they graze quickly. Actually, neither is true.

Models must compensate for such large differences in zooplankton grazing by making additional assumptions about how fast phytoplankton grow and how fast zooplankton die. Together, these differences can be balanced in a way that allows most models simulate the actual amount of carbon consumed by phytoplankton and transferred to the deep sea.

However, that’s only because we can observe what those values ​​should be. We can then adjust the models until we are sure they get the correct answer.

However, although our best models can admirably recreate the ocean today, they do so for different reasons and with dramatically different assumptions about the role of zooplankton.. This means that these models are built with fundamentally different machinery. When used to test future emissions scenarios, they will project fundamentally different results.

We cannot know which projections are correct unless we know the true role of zooplankton.