Rising temperatures can delay the arrival of spring

Warmer temperatures are often expected to make plants start growing earlier; that is what scientists have long assumed. But ITC researcher Siqi Shi tells a more complex story. Analyzing decades of satellite data across Africa and beyond, she shows that once temperatures exceed a plant's optimal growth threshold, further warming delays rather than advances the start of the growing season.

Plants are not passive bystanders in a warming world. Through their seasonal cycles of growth and dormancy, they regulate how much carbon dioxide ecosystems absorb and how land and atmosphere interact. In regions where temperatures are already high, plants may start growing later, not earlier, as previously thought.

The continent that broke the pattern

For decades, rising temperatures pushed the start of the growing season earlier across the Northern Hemisphere, and this has become a cornerstone of climate science. Satellite observations confirmed it at the continental scale. Then, tropics and subtropics, such as Africa, told a different story.

Using long-term satellite data spanning the entire continent, Shi mapped when vegetation starts growing, peaks, and dies back across Africa's diverse landscapes. The result was striking: across much of the continent, the start of the growing season has been shifting later, not earlier. "We started out just wanting to study phenology in Africa. Then we saw the delayed start of the growing season, and everything changed," says Shi.

Tug-of-war with precipitation

More rain tends to bring the growing season forward, and parts of Africa have seen slightly more precipitation over recent decades. But the data show that warming is winning. A one-degree rise in temperature in the months before the growing season delays its start by nearly five days on average. An extra 10 millimeters of rain advances it by less than two. The result, across most of the continent, is a season that keeps shifting later.

Why timing matters

The start of the growing season marks not only when vegetation begins to grow, but also when ecosystems start to absorb carbon from the atmosphere. Accurately identifying this timing is therefore crucial for estimating how much carbon land ecosystems can store. This matters for the models scientists use to simulate the global carbon cycle. Many land surface and ecosystem models assume a fixed threshold to trigger the start of the growing season and assume that warmer temperatures generally advance plant growth.

If this assumption does not hold in some regions, these models may misrepresent when vegetation becomes active and starts taking up carbon from the atmosphere. The predictions of ecosystem carbon uptake and climate-vegetation interactions may become inaccurate. This can affect projections of the global carbon cycle and future climate change. The findings are also relevant beyond climate science. Farmers often rely on seasonal climate information to decide when to sow crops, irrigate fields, or apply fertilizer. If warming shifts plant growth in unexpected ways, agricultural planning may also need to adapt.

The optimal temperature

The key to understanding Africa's delayed spring lies in a concept called vegetation optimal temperature: the temperature at which a plant grows best. Below that threshold, warming promotes growth and advances the start of the growing season. But once temperatures approach or exceed it, the relationship inverts: additional warming slows growth and pushes the season later.

Across much of Africa, temperatures during the growing season are already high. Warming does not help, it hinders. And Africa is not alone. The same mechanism applies wherever vegetation is already operating near or beyond its heat limit, from subtropical regions to parts of Asia and South America. It is a global pattern hiding behind a continental case study.

Shi says, "This isn't just an Africa story. Wherever plants are already growing near their heat limit, further warming will delay, not advance, the start of the growing season. Many phenology models don't account for this, and that matters for how we predict carbon storage, drought, and food security."

Satellites as witnesses

Shi's findings would not have been possible without satellite data. Ground measurements can track individual sites, but satellites can observe entire continents over decades. Her research also exposed a subtler problem. Scientists use two types of satellite measurements to track plant growth: one measuring the greenness of the canopy, the other measuring chlorophyll fluorescence (a faint light signal plants emit during photosynthesis), used as a more direct measure of how actively they are growing. The two do not always agree. Shi shows that differences in sunlight largely explain the mismatch, a finding that matters beyond her own work.

Provided by University of Twente