The rainy season of the Amazon rainforest begins two to three months before other tropical areas. But why does it begin so early? The answer is that the Amazon triggers its own rainy season — all by itself. There have been theories, but until recently, it’s been a mystery, but new research conducted by scientists at UCLA and NASA, and published in the journal Proceedings of the National Academy of Sciences (PNAS), has revealed the reason behind this fascinating phenomenon.

The rainy season in the majority of the tropics is brought about by shifting monsoon winds and trade winds, and the shift of these winds doesn’t occur until December or January. The Amazon’s rainy season, however, begins in October. This time gap suggests that wind shifts probably aren’t the cause of the Amazon’s wet season, explains NASA.

In 2004, UCLA scientist Rong Fu, one of the leaders of the recent research, published a paper that suggests that the cause of the Amazon’s ability to trigger its own rainy season and produce moisture to create low-level clouds and rain might be due to transpiration. Transpiration is the process of water evaporation from plant leaves.

While Fu’s original research on transpiration and the Amazon’s wet season was helpful, “We didn’t have hard evidence,” she said. "We speculated that the moisture came from vegetation because satellite measurements showed the vegetation became greener at the end of the dry season.” The observation of greener plants was certainly a step in the right direction to unlocking the mystery, but it wasn’t enough.

Seeing the evidence

Now, thanks to new satellite data technology developed by John Warden of NASA's Jet Propulsion Laboratory in Pasadena, California, Fu and her team were given the possibility to better determine if and how transpiration is what allows for the Amazon’s ability to trigger its own rainy season and create low-level clouds.

With the new technology, the scientists were able to study isotopes in the water vapor above the Amazon. Specifically, Fu and her colleagues examined the role of a hydrogen isotope called deuterium, which combines with oxygen to produce heavy water. Since deuterium is a heavy isotope, water molecules that contain deuterium have a harder time evaporating than if the isotope were lighter. Because of this, there's less deuterium in water vapor then there is in liquid water — meaning that, for example, water vapor that has evaporated from the ocean has less deuterium than the ocean itself. In the case of transpiration, the plants and trees are able to absorb water from the soil without any change in deuterium levels.

Interestingly enough, the water vapor above the Amazon contains a high amount of deuterium — a higher level than would be present if the water vapor came from the ocean. This level was “too high to be explained by water vapor from the ocean,” says Fu, as reported in the journal Science. The high deuterium levels gave Fu and her team strong evidence that the vapor came from the trees themselves and that transpiration is what allows the Amazon to trigger its rainy season.

The results are certainly fascinating, but they also give us insight into the dangers of deforestation and how deforestation affects the Amazon ecosystem and our ecosystem as a whole. If it’s the trees that kick off the rainy season in the Amazon, then fewer trees due to deforestation translates to less rain all around. Less rain makes for a longer dry season, and that's detrimental. "The fate of the southern Amazon rainforest depends on the length of the dry season, but the length of the dry season also depends on the rainforest,” Fu says.

So even though we’ve been given new insights into the inner workings of a wondrous natural phenomenon, this discovery also gives us the chance to think about the way our actions affect our environment.