Earth's Seasons Are Out of Sync, Scientists Discover From Space

The annual turning of the seasons – winter, spring, summer and autumn – is often treated as a fixed, universal pattern.

Our new study in Nature, which uses a novel way to observe seasonal growth cycles from satellites, shows that this idea is far too simplistic.

We offer an unprecedented, close-up view of the seasonal rhythms of Earth’s land ecosystems. In doing so, we identify global “hotspots” of seasonal asynchrony – areas where the timing of seasonal cycles is mismatched between places that are close together.

We then demonstrate that these timing differences can carry unexpected ecological, evolutionary and even economic implications.

Watching seasonal growth cycles from space

The seasons shape life’s tempo. Living organisms, humans included, tune the timing of yearly activities to make the most of resources and conditions that change over the course of the year.

The discipline that examines this timing is known as “phenology”, and it is one of humanity’s oldest ways of observing the natural world. Today, however, phenology can also be monitored from space.

Thanks to decades of archived satellite images, we can apply computing methods to improve our understanding of seasonal plant growth. Yet the techniques most commonly used often assume that seasonal cycles are straightforward and that growing seasons are clearly defined.

This assumption works well across much of Europe, North America and other high-latitude regions where winters are pronounced. In the tropics and in arid landscapes, though, these approaches can perform poorly. In such places, satellite-derived signals of plant growth may shift only subtly across the year, without distinct, neatly bounded growing seasons.

Surprising global patterns in plant growth timing

Using a new analysis applied to 20 years of satellite imagery, we produced an improved global map of when plant growth cycles occur. Alongside familiar patterns – such as spring arriving later at higher latitudes and elevations – we also detected several unexpected ones.

One striking pattern appears across Earth’s five Mediterranean climate regions, characterised by mild, wet winters and hot, dry summers. These occur in California, Chile, South Africa, southern Australia and the Mediterranean Basin.

All of these regions share a “double peak” seasonal signature, previously documented in California, because forest growth cycles typically reach their peak around two months later than those of other ecosystems. They also display sharp contrasts in the timing of plant growth compared with nearby drylands, where summer rainfall is more frequent.

Spotting hotspots of seasonal asynchrony

This intricate patchwork of seasonal activity helps explain a key outcome of our study: Mediterranean climates and their adjacent drylands form hotspots of out-of-sync seasonal activity. Put simply, they are places where nearby locations can experience seasonal cycles that occur at markedly different times.

Take, for instance, the clear contrast between Phoenix, Arizona (which receives similar amounts of rainfall in winter and summer) and Tucson, just 160 km away (where the summer monsoon delivers most of the rain).

Other hotspots worldwide are found mainly in tropical mountain regions. The detailed, mismatched seasonal patterns we see there may be linked to the complex ways mountains shape airflow, which in turn controls local seasonal rainfall and cloud formation.

Although these processes remain poorly understood, they may be central to explaining why species are distributed as they are in these exceptionally biodiverse parts of the world.

Seasonality, biodiversity hotspots and evolutionary consequences

Pinpointing the world’s regions with out-of-sync seasonal patterns was our original aim. Our discovery that many of these areas overlap with Earth’s biodiversity hotspots – regions with unusually high numbers of plant and animal species – may well be more than coincidence.

Where plant growth cycles are out of sync across short distances, the seasonal supply of resources is likely to be out of sync as well. That, in turn, would influence the seasonal reproductive timing of many species, with potentially far-reaching ecological and evolutionary consequences.

One possible consequence is that populations whose reproductive cycles are out of step would be less likely to interbreed. Over time, those populations would be expected to become more genetically distinct, and might ultimately even separate into different species.

If this occurred for even a small fraction of species at any moment, then over long timescales these regions could generate very high levels of biodiversity.

Back down to Earth

We do not yet know whether this process has truly been unfolding in nature. However, our research represents an initial move towards answering that question.

We show that, across a broad range of plant and animal species, our satellite-based map anticipates pronounced on-the-ground differences in the timing of plant flowering, as well as differences in genetic relatedness among nearby populations.

Our map also captures the intricate geography of coffee harvests in Colombia. There, coffee farms separated by a day’s drive across mountainous terrain can have reproductive cycles that are as out of sync as if they were located in opposite hemispheres.

Understanding how seasonal patterns vary across space and time is not only crucial for evolutionary biology. It also underpins our understanding of the ecology of animal movement, the impacts of climate change on species and ecosystems, and even the geography of agriculture and other human activities.

Want to know more? You can explore our findings in greater depth via this interactive online map, which we also include below.

Drew Terasaki Hart, Ecologist, CSIRO

This article is republished from The Conversation under a Creative Commons licence. Read the original article.