Read the following text and fill in each gap with a suitable word based on the information of the text:

In a speech at the 90th science congress, internationally acknowledged scientist and former Indian President Mr. APJ Abdul Kalam mentioned a very important aspect of this mankind's future energy crisis. He pointed out that the era of wood and bio-mass has almost come to an end. The age of oil and natural gas would soon be over within the next few decades. Massive burning of world's coal reserves may lead to a worldwide ecological disaster because coal burning emits the highest amount of carbon in the atmosphere. So according to Kalam, the only solution that mankind can look to is the massive use of solar energy in future because it has some advantages over other forms of renewable energies.

Now, why has Kalam put so much importance to the issue of energy? The energy sources have always been a major factor of change throughout history. The world's petroleum consumption has increased from 3 billion barrels annually in 1930 to 37 billion barrels annually today. In the next quarter century, the world's population is expected to be about 8 billion which is 30 percent higher than today. Developing countries will grow their economics about two times faster than industrialized countries.

Global economic growth is expected to continue at 3 percent per year. Consequently, the global demand for energy will grow at about 1-7 percent per year on an average. It indicates a 50-percent rise of energy consumption by 2030. If the world's daily petroleum consumption is 100 million barrels now, it will rise to 150 million barrels by that time. The present reserve of hydro- carbon energy resources is limited and it will not be sufficient to meet the future energy challenges of the world. Hence, leading industrial counties have taken initiatives to use alternative energy sources mainly known as green or renewable energy sources. The bottom line of Kalam's speech indicates the concern of mankind in the 21st century.