The Main Cause of the Global Warming

Global Warming
Recently, the earth is getting hotter and hotter, a great amount of glacier is now melting rapidly. The water level is also rising. For that influences, some country is in severe danger, submerged with the ocean like Tuvalu Island. They are now have to leave their country because of Global warming. Not only human being in danger, but also whole creatures suffer from huge catastrophe. For example, polar bear is now in extinction, with the lost of their habitats. Also, a variety of needle-leaf plants, which cannot adapt to hot weather, is also in extinction.
Then why Global warming is happened? What's the main cause of global warming? there are two main theories, proving the cause of that catastrophe. Some scientists insists that global warming is occurred by human activities. The other claimed that global warming is happened naturally, caused by Earth's interior activity : the transition of core.
First of all, scientists in Boston University concluded that global warming is mainly caused by human activities. Human have devastated the nature since industrial revolution. Countless cars and factories emit a billion tons of carbon dioxide to the atmosphere, which is impossible to purify by force of nature. If the green house gases, surrounding the earth surface, is increased, then it holds the light from the sun, inducing the green house effect. Furthermore, It destroys the Ozone layer, protecting the earth from harmful ultraviolet rays which is fatally harmful to the nature. In this regard, they believe that global warming is manmade disaster.
On the other hand, research team in Havard University insisted that global warming is natural phenomenon. As we know, the earth have a geographical cycle, interglacial and glacial epoch. When the earth's core is activated, the crust of the earth and the ocean is heated. Then, heated ocean emit a tons of carbon dioxide to the atmosphere. To explain specifically, we already learned about that principle in science class. Cold water hold gases, while Hot water emit them. Comparing the amount of carbon dioxide between the human activity and natural process, they concluded that the manmade greenhouse gases is practically nothing. It means that the main cause of the global warming is nature.
In short, there are two theories, confirming the main cause of global warming. Some scientists insist that it is the manmade catastrophe. The other persists that it is natural phenomenon. In my opinion, I believe that global warming is the results of both human activity and nature.