CAUSES OF CLIMATE CHANGE

How is the global earth system changing? 

http://science.nasa.gov/earth-science/big-questions/is-the-global-earth-system-changing-and-what-are-the-consequences/ 

Earth is currently in a period of warming. Over the last century, Earth's average temperature rose about 1.1°F (0.6°C). In the last two decades, the rate of our world's warming accelerated and scientists predict that the globe will continue to warm over the course of the 21st century. Is this warming trend a reason for concern? After all, our world has witnessed extreme warm periods before, such as during the time of the dinosaurs. Earth has also seen numerous ice ages on roughly 11,000-year cycles for at least the last million years. So, change is perhaps the only constant in Earth's 4.5-billion-year history.
Scientists note that there are two new and different twists to today's changing climate: (1) The globe is warming at a faster rate than it ever has before; and (2) Humans are the main reason Earth is warming. Since the industrial revolution, which began in the mid-1800s, humans have attained the magnitude of a geological force in terms of our ability to change Earth's environment and impact its climate system.
Since 1900, human population doubled and then doubled again. Today more than 6.5 billion people inhabit our world. By burning increasing amounts of coal and oil, we drove up carbon dioxide levels in the atmosphere by 30 percent. Carbon dioxide is a "greenhouse gas" that traps warmth near the surface.
Humans are also affecting Earth's climate system in other ways. For example, we transformed roughly 40 percent of Earth's habitable land surface to make way for our crop fields, cities, roads, livestock pastures, etc. We also released particulate pollution (called "aerosols") into the atmosphere. Changing the surface and introducing aerosols into the atmosphere can both increase and reduce cloud cover. Thus, in addition to driving up average global temperature, humans are also influencing rainfall and drought patterns around the world. While scientists have solid evidence of such human influence, more data and research are needed to better understand and quantify our impact on our world's climate system


http://www.climatechange.gc.ca/default.asp?lang=en&n=65CD73F4-1 

Climate change is a long-term shift in weather conditions identified by changes in temperature, precipitation, winds, and other indicators. Climate change can involve both changes in average conditions and changes in variability, including, for example, extreme events.
The earth's climate is naturally variable on all time scales. However, its long-term state and average temperature are regulated by the balance between incoming and outgoing energy, which determines the Earth’s energy balance. (Learn more about the Earth’s climate system). Any factor that causes a sustained change to the amount of incoming energy or the amount of outgoing energy can lead to climate change. As these factors are external to the climate system, they are referred to as ‘climate forcers’, invoking the idea that they force or push the climate towards a new long-term state – either warmer or cooler depending on the cause of change. Different factors operate on different time scales, and not all of those factors that have been responsible for changes in earth’s climate in the distant past are relevant to contemporary climate change. Factors that cause climate change can be divided into two categories ­- those related to natural processes and those related to human activity. In addition to natural causes of climate change, changes internal to the climate system, such as variations in ocean currents or atmospheric circulation, can also influence the climate for short periods of time. This natural internal climate variability is superimposed on the long-term forced climate change. 

Natural Causes

The Earth’s climate can be affected by natural factors that are external to the climate system, such as changes in volcanic activity, solar output, and the Earth's orbit around the Sun. Of these, the two factors relevant on timescales of contemporary climate change are changes in volcanic activity and changes in solar radiation. In terms of the Earth’s energy balance, these factors primarily influence the amount of incoming energy. Volcanic eruptions are episodic and have relatively short-term effects on climate. Changes in solar irradiance have contributed to climate trends over the past century but since the Industrial Revolution, the effect of additions of greenhouse gases to the atmosphere has been about ten times that of changes in the Sun’s output. 

Solar Variability and Terrestrial Climate

In the galactic scheme of things, the Sun is a remarkably constant star.  While some stars exhibit dramatic pulsations, wildly yo-yoing in size and brightness, and sometimes even exploding, the luminosity of our own sun varies a measly 0.1% over the course of the 11-year solar cycle. 
There is, however, a dawning realization among researchers that even these apparently tiny variations can have a significant effect on terrestrial climate. A new report issued by the National Research Council (NRC), "The Effects of Solar Variability on Earth's Climate," lays out some of the surprisingly complex ways that solar activity can make itself felt on our planet.

http://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate/  


 These six extreme UV images of the sun, taken by NASA's Solar Dynamics Observatory, track the rising level of solar activity as the sun ascends toward the peak of the latest 11-year sunspot cycle  



 

What's climate change got to do with it?

Quite simply, the more solar radiation bombarding Earth at any given time, the warmer the planet should get. So Earth's place in each of these cycles should have a measurable effect on long term climate trends — and it does. But that's not all. Another factor has to do with which hemisphere happens to be receiving the heaviest bombardment. This is because land warms faster than oceans do, and the Northern Hemisphere is covered by more land and less ocean than the Southern Hemisphere is.