"The data reveal a striking change in the carbon cycle in the Northern Hemisphere across seasons, where in the spring there's a dramatic uptake of carbon by terrestrial plants," said one of the five papers in Science.
This cycle, coupled with the continual emissions from fossil fuel burning over China, Europe and the southeast United States, means carbon levels reach a seasonal high in April in the northern hemisphere, it said.
Another study in Science found that the ocean warming phenomenon known as El Nino resulted in far more carbon release in the tropics than in previous years.
"Lower precipitation in South America and increased temperatures in Africa were key drivers" of this change, it added.
Since climate change is expected to bring less rain to South America and higher temperatures to Africa by the end of the century, researchers warn the trend will get worse in the tropics, which have traditionally served as a buffer for fossil fuel emissions because they absorb so much carbon.