All the options you provided are effects or consequences of global warming, but they emphasize different aspects of the issue. Here's a brief explanation of each:
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Increasing greenhouse gas concentration in air: This is a primary driver of global warming, as more greenhouse gases (like CO2, methane, etc.) trap heat in the atmosphere.
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Rising sea levels; over a period of time: This is a well-documented effect of global warming, primarily due to the melting of polar ice and glaciers, as well as the thermal expansion of seawater as it warms.
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Changing seasons every year: While seasons change due to Earth’s axial tilt and orbit, global warming can alter seasonal patterns, leading to earlier springs, irregular weather patterns, or shifts in climatic zones.
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Increasing carbon dioxide in the atmosphere: This is both a cause and an effect of global warming. Human activities, especially the burning of fossil fuels, increase CO2 in the atmosphere, contributing to the greenhouse effect and further warming.
If you are looking for the effect of global warming among the options provided, "rising sea levels; over a period of time" is a clear and direct consequence of global warming. However, all options represent aspects related to global warming.