Humans impact the physical environment in many ways: overpopulation, pollution, burning fossil fuels, and deforestation. Most notably, human impacts have contributed to climate change. Scientists attribute the global warming trend observed since the mid-20th century to the human expansion of the "greenhouse effect" — warming that results when the atmosphere traps heat radiating from Earth toward space. Certain gases in the atmosphere block heat from escaping. Gases that contribute to the greenhouse effect include: Water vapor, carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFCs). The effects of this are difficult to predict but some effects are more certain:

-On average, Earth will become warmer. Some regions may welcome warmer temperatures, but others may not.
-Warmer conditions will probably lead to more evaporation and precipitation overall, but individual regions will vary, some becoming wetter and others dryer.
-A stronger greenhouse effect will warm the oceans and partially melt glaciers and other ice, increasing sea level. Ocean water also will expand if it warms, contributing further to sea level rise.
-Meanwhile, some crops and other plants may respond favorably to increased atmospheric CO2, growing more vigorously and using water more efficiently. At the same time, higher temperatures and shifting climate patterns may change the areas where crops grow best and affect the makeup of natural plant communities.

Not all human impacts are negative. Ways in which people positively affect ecosystems around the world include:
-Recycling
-Establishing wildlife preserves and parks
-Creating green, open space laws
-Doing reforestation
-Creating environmental regulations

The answer is "carbon monoxide."

Humans impact the physical environment in many ways: overpopulation, pollution, burning fossil fuels, and deforestation. Most notably, human impacts have contributed to climate change. Scientists attribute the global warming trend observed since the mid-20th century to the human expansion of the "greenhouse effect" — warming that results when the atmosphere traps heat radiating from Earth toward space. Certain gases in the atmosphere block heat from escaping. Gases that contribute to the greenhouse effect include: Water vapor, carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFCs). The effects of this are difficult to predict but some effects are more certain:

-On average, Earth will become warmer. Some regions may welcome warmer temperatures, but others may not.
-Warmer conditions will probably lead to more evaporation and precipitation overall, but individual regions will vary, some becoming wetter and others dryer.
-A stronger greenhouse effect will warm the oceans and partially melt glaciers and other ice, increasing sea level. Ocean water also will expand if it warms, contributing further to sea level rise.
-Meanwhile, some crops and other plants may respond favorably to increased atmospheric CO2, growing more vigorously and using water more efficiently. At the same time, higher temperatures and shifting climate patterns may change the areas where crops grow best and affect the makeup of natural plant communities.

Not all human impacts are negative. Ways in which people positively affect ecosystems around the world include:

-Recycling
-Establishing wildlife preserves and parks
-Creating green, open space laws
-Doing reforestation
-Creating environmental regulations