Natural Greenhouse Effect
The Earth receives heat from the Sun. About half of the Sun’s energy is absorbed by the Earth’s surface, while around 20% of the energy is absorbed by the atmosphere and 30% is reflected by clouds and the Earth’s surface back into space.
As the Earth’s surface warms, energy is emitted back into the atmosphere in a similar way that the hob of an electric cooker radiates heat. But if that’s all that happened, the Earth’s surface would be frozen, with an average temperature of around -18 °C — too cold to support life.
Instead, greenhouse gases in the Earth’s atmosphere absorb some of the outgoing energy and return part of it to the Earth’s surface. These gases are only a small proportion of the atmosphere but they act like a blanket by trapping some of the heat. The greater the concentration of these greenhouse gases, the more effectively they return energy back to the Earth’s surface.
Scientists explained the heat-trapping effects of greenhouse gases more than 150 years ago. We now know that any changes in the amount of greenhouse gases in the atmosphere will affect the Earth's temperature.
the main greenhouse gases are
- Water vapour (H2O)
- Carbon dioxide (CO2)
- Methane (CH4)
- Nitrous oxide (N2O)
- Ozone (O3)
Enhanced Greenhouse Effect
Human activities like burning coal, oil and gas, deforestation and some agricultural practices have released huge quantities of greenhouse gases into the atmosphere.
This increase in greenhouse gases has led to an imbalance in the energy cycle that has affected the water cycle, atmospheric circulation and ocean currents, leading to changes in weather and climate. This is sometimes called the enhanced greenhouse effect.
In fact, the observed rise in atmospheric CO2 represents only half of that actually released, showing that some CO2 has been absorbed by our plants and oceans. Without this, it is possible the enhanced greenhouse effect would be even greater.
Observations of temperature, rainfall, humidity, arctic sea-ice, glaciers and sea-level over the last century are consistent with a warming planet. The evidence indicates that it is very likely that most of the warming since the mid-20th century is due to the observed increase in human emissions of greenhouse gases.
Water vapour is a really important greenhouse gas. Unlike other greenhouse gases, such as CO2 and methane, our activities aren’t really influencing its concentration in our atmosphere. However, scientists need to consider water vapour’s influence on the climate system as it plays a vital role in how much global temperatures might rise in the future.