Over the past century the concentration of greenhouse gases in the atmosphere has waived up and down increased then unchanged decreased

Answer 1

Over the past century, the concentration of greenhouse gases in the atmosphere has generally increased, particularly due to human activities such as burning fossil fuels, deforestation, and industrial processes. The levels of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have risen significantly since the late 19th century, contributing to global warming and climate change. While there can be short-term fluctuations or variations in concentrations, the long-term trend has been an overall increase.