Global warming

How do scientist know that the amount of carbon dioxide in our atmosphere has risen? Carbon dioxide is a measurable greenhouse gas.scientist cab determine how much has accumulated in the atmosphere from measurements of air that's up to a million years old, trapped within air bubbles in icecores, and from air samples taken from the 1950's onwards. Thus gives them a reliable long and short term record.               Human activity: most scientists have concluded that human activities are the main cause for the world warming up. Some activities increase the concentration of some greenhouse gases in the atmosphere most importantly carbon dioxide and methane. In the past human activities have only had a minor effect. But as technology and civilisation have advabced, activities causing greenhouse gas emissions have been increasing. These include transport, building, agriculture and the way we generate power. At the same time position such as smog has increased and some of the pollutants had a cooling effect but thus was reduced in the 1970's in Europe and North America by health laws. The biggest increases in greenhouse gas emissions have occurred in the last 50 years- at the same time as the fastest increases in temperature. Thus and other evidence, including direct measurements and known physical laws, have led scientists to conclude that the two are indeed closely linked.      However could changes in the sun be the cause? Chnages in the suns output do affect the Earth's temperature. The suns output is slightly higher when there are more sunspots and lower when there are fewer. During a period of more sunspots in the middle ages the climate warmed up slightly. Temperatures in Europe rose,but its not clear if the effect was worldwide. Sunspot activity goes up and down in cycles of about 11 years. But once averaged out,  the Sun's output has changed little in the past 50 years.                                                                                                                                                El Niño: The Earth's climate system includes natural cycles which affect how heat is spread across the globe. These both warm and cool the atmosphere, and cab cause impacts ranging from floods to fire. Of these, the El Niño cyclenhad the most effect on global tempture during the past century. In 1998 an especially strong El Niño event occurs, causing the world the warm. This had knock on effect