Earth's seasons observed from space

 Earth's seasons aren't the same everywhere, Scientists discover from space

The annual clock of the seasons, winter, spring, summer, autumn, is often taken as a given. But our new study in Nature, using a new approach for observing seasonal growth cycles from satellites, shows that this notion is far too simple. A new study about Nature reveals that Earth's seasons don't actually line up the same everywhere. By looking at 20 years of satellite data, researchers found big differences in when plants grow, even between places which are pretty close together. So, the idea of what everyone shares the same spring or fall? Not really true. We present an unprecedented and intimate portrait of the seasonal cycles of Earth's land-based ecosystems. This reveals "hotspots" of seasonal asynchrony around the world, regions where the timing of seasonal cycles can be out of sync between nearby locations. We then show these differences in timing can have surprising ecological, evolutionary, and even economic consequences.

By applying a new analysis to 20 years of satellite imagery, we made a better map of the timing of plant growth cycles around the globe. Alongside expected patterns, such as delayed spring at higher latitudes and altitudes, we saw more surprising ones too. Average seasonal cycles of plant growth around the world. Each pattern varies from its minimum (tan) to its maximum (dark green) throughout the year. One surprising pattern happens across Earth's five Mediterranean climate regions, where winters are mild and wet and summers are hot and dry. These include California, Chile, South Africa, southern Australia and the Mediterranean itself. These regions all share a "double peak" seasonal pattern, previously documented in California, because forest growth cycles tend to peak roughly two months later than other ecosystems. They also show stark differences in the timing of plant growth from their neighbouring dry lands, where summer precipitation is more common.

The seasons set the rhythm of life. Living things, including humans, adjust the timing of their annual activities to exploit resources and conditions which fluctuate through the year. The study of this timing, known as "phenology", is an age-old form of human observation of nature. But today, we can also watch phenology from space. The average seasonal growth cycles of Earth's land-based ecosystems, estimated from 20 years of satellite imagery gives a different results. With decades-long archives of satellite imagery, we can use computing to better understand seasonal cycles of plant growth. However, methods for doing this are often based on the assumption of simple seasonal cycles and distinct growing seasons. This works well in much of Europe, North America and other high-latitude places with strong winters. However, this method can struggle in the tropics and in arid regions. Satellite-based estimates of plant growth can vary subtly throughout the year here, without clear-cut growing seasons.

The biggest "out-of-sync" hotspots showed up in Mediterranean climates and tropical mountains, think California, Chile, South Africa or the Mediterranean itself. These timing gaps can shape which species thrive by changing how plants reproduce and spread genes. This even matches up with the complex geography of Colombia's coffee harvests, where nearby farms can have out-of-sync cycles. This complex mix of seasonal activity patterns explains one major finding of this work: the Mediterranean climates and their neighbouring dry lands are hotspots of out-of-sync seasonal activity. In other words, they are regions where the seasonal cycles of nearby places can have dramatically different timing. Consider, for example, the marked difference between Phoenix, Arizona (which has similar amounts of winter and summer rainfall) and Tucson only 160 km away (where most rainfall comes from the summer monsoon). Other global hotspots occur mostly in tropical mountains. The intricate patterns of out-of-sync seasons we observe there may relate to the complex ways in which mountains can influence airflow, dictating local patterns of seasonal rainfall and cloud. These phenomena are still poorly understood, but may be fundamental to the distribution of species in these regions of exceptional biodiversity.

Scientists tracked "phenology" basically, when plants and animals do their seasonal things, using global satellite imagery. Older methods mostly worked for places with clear-cut winters and summers but missed all the weird timing in tropical and dry regions. This study created the most detailed map yet of how seasonal events play out around the world. Identifying global regions where seasonal patterns are out of sync was the original motivation of the work. And final finding overlap with many of Earth's biodiversity hotspots, places with large numbers of plant and animal species, may not be a coincidence. In these regions, because seasonal cycles of plant growth can be out of sync between nearby places, the seasonal availability of resources may be out of sync, too. This would affect the seasonal reproductive cycles of many species, and the ecological and evolutionary consequences could be profound. One such consequence is that populations with out-of-sync reproductive cycles would be less likely to interbreed. As a result, these populations would be expected to diverge genetically, and perhaps eventually even split into different species. If this happened to even a small percentage of species at any given time, then over the long haul these regions would produce large amounts of biodiversity.

For a wide range of plant and animal species, satellite-based map predicts stark on-ground differences in the timing of plant flowering and in genetic relatedness between nearby populations. Understanding seasonal patterns in space and time isn't just important for evolutionary biology. It is also fundamental to understanding the ecology of animal movement, the consequences of climate change for species and ecosystems, and even the geography of agriculture and other forms of human activity around the world.