The researchers used satellite data and computer models to study why certain gases in the Earth's upper atmosphere are changing differently in the Northern and Southern Hemispheres. They found that the circulation of air in the stratosphere is causing these differences. In the Northern Hemisphere, more nitrogen and methane are moving from the upper to the lower stratosphere, leading to decreases in those gases. Meanwhile, in the Southern Hemisphere, more of these gases are moving from lower to middle latitudes, causing increases. The same circulation patterns are also affecting the levels of hydrogen chloride, with opposite trends in each hemisphere. Understanding these processes can help predict future changes in the Earth's stratosphere.