Carbon dioxide and methane are greenhouse gases. If their levels in the atmosphere keep rising, the Earth's average temperature is likely to rise. This is global warming, and it can cause climate change.
Possible effects include the melting of ice caps and glaciers, which, along with the thermal expansion of warming sea water, raises sea level and causes flooding of low-lying land. Weather patterns may change, with more extreme events such as storms, droughts and heatwaves. Rainfall may change, which can affect crop growth and food supply. Some habitats will change, so some species may move or die out. Small changes in average temperature matter because they affect the whole planet.
These effects can be mitigated, meaning reduced. Burning less fossil fuel, and using renewable energy sources such as wind and solar power, cuts carbon dioxide emissions. Planting trees takes in carbon dioxide. Carbon capture and storage stops carbon dioxide reaching the air. Methane can be reduced by capturing it from landfill sites and by changing farming practices. People can also cut their carbon footprint, which is the total carbon dioxide and methane released by an activity or a product over its life.
These actions have costs and need agreement between countries, so there are difficult choices about risk, scale and the environment.