Microscopic organisms in the ocean called phytoplankton use the sun's energy to convert carbon dioxide (CO2), nutrients and water into organic matter, just as plants do on land. This organic matter is grazed upon by tiny animals called zooplankton that are found throughout the global ocean. Marine zooplankton are so abundant that the total weight of their global population greatly exceeds that of the ~8 billion humans alive on Earth today. Like all animals, zooplankton produce vast quantities of faecal matter that they eject into the surrounding environment. Some of this waste sinks down into the abyss, carrying with it carbon that was once in the atmosphere as CO2. Any faecal carbon that reaches the deep ocean may be locked away down there for 100's or even 1000's of years. The process of exporting carbon in this way occurs on such a scale that it plays a fundamental role in global climate regulation, keeping our planet cool by slowing the rate at which CO2 accumulates in our atmosphere.

Zooplankton are cold-blooded, and as such, their physiological rates increase as their environment warms. By contrast, the body size of zooplankton decreases with warming, although the mechanism underlying this phenomenon remains uncertain. Indeed, there are many gaps in our understanding of how temperature affects zooplankton physiology. For example, does the rate at which they can capture food increase at the same rate at which their demand for energy increases with warming? If it does, perhaps they will simply eat their way out of the climate crisis? But what if it doesn't? Continued ocean warming may then result in zooplankton having to use more and more of their food to meet the temperature-driven increase in their energy demands, leaving less and less for growth and reproduction. Does this situation get worse if the amount of food available to zooplankton decreases with ocean warming? And do different sized individuals respond differently to temperature? Our incomplete understanding of the interplay between temperature, food supply and zooplankton body size means that we cannot reliably predict their response to ocean warming. Indeed, most global models of the ocean ecosystem that are used to help predict future climate assume that these aspects of zooplankton physiology are fixed, with no sensitivity to warming. We therefore have only limited confidence in our ability to forecast how the zooplankton contribution to global climate regulation via ocean carbon storage will change as the ocean warms throughout the 21st century.

Our project, C-QWIZ, will determine how zooplankton of different sizes respond to increasing temperatures at different levels of food. In doing so, we will fill many of the knowledge gaps in our fundamental understanding of their physiological response to climate change. The C-QWIZ team is uniquely placed to translate this new understanding into existing mathematical models of the global ocean ecosystem; we will be the first to mechanistically assess how global warming affects zooplankton-mediated ocean carbon storage throughout the 21st century. Our chosen model is used by scientists around the world to forecast how Earth's future climate will change. These forecasts are used by politicians and policy makers to decide on how best to manage the future of our planet. Improving these models therefore ensures that our science delivers real and lasting change for the benefit of all society.