By using various methods of chemistry, geology, and even astronomy, past climate variations are well known. These include the relatively periodic ice ages of the past 2-million years as well as more exotic climates from the Cretaceous and other eras. On such long timescales the main reasons for climate changes must be linked to changes in the sunlight reaching Earth (insolation), major shifts in ocean heat transport, or "external" forcing like volcanism or meteor impacts. More recently, human-induced changes (anthropogenic) are likely a strong climate forcing. Much research has focused on quantifying Earth's climate and its variability over the past 30 years or more. It has been shown that the globally averaged surface temperature is now warmer than it has been for at least 150 years. The trend in surface temperature is remarkably well correlated with a trend in atmospheric carbon dioxide. The current trends are increasing, and the warmest years on record are in the last decade. IPCC
In fact, according to data compiled by NOAA, 2012 ranks 10th warmest since records began in 1880 . The global surface temperature was estimated to be 14.47°C (0.57°C above the 20th Century average of 13.9°C). Amazingly, 2012 was the 36th consecutive year that the yearly global temperature was above average, meaning that more than half the people in the world have never experienced a (globally) average year (median age is 28.4 years, ).
The global average surface temperature continues to rise, but not every year is the warmest year ever. There are variations in the global average surface temperature. These variations are caused by small deviations in the radiative forcing or fluctuations in the rate of heat absorption by the deep ocean. The long-term trend provides a measure of the rate of warming. Putting a linear trend line through the observations of the late 20th Century and early 21st Century provides an estimate of 0.13 °C [0.10 to 0.16 °C] per decade according to the IPCC AR4.
Earth is now absorbing 0.85±0.15 W m-2 more energy from the Sun than it is emitting to space. (Hansen et al, 2005, Science vol 308)
The map at right shows an expression of the directly measured temperature change. It shows the temperature anomaly map averaged over the ten years spanning 2000 to 2009 in Kelvin (a centigrade scale, just like Celsius). These anomalies are with respect to the 1961 to 1990 base period. The measurements that go into the HadCRUT3 data set include observations over land from more than 3000 weather stations on at least a monthly basis. For ocean areas, ship-based measurements are used, details are included in published papers (Jones & Moberg 2003 and Rayner set al. 2003).
The time series from the HadCRUT3 data set is also shown at right. While temperature anomalies are fairly uniform in the late 19th Century, the 20th Century shows a strong upward trend. The trend is divided into an early rise from around 1900 to about 1940 followed by a period of little warming (and some cooling) until the 1970s and then a rapid rise from before 1980 until present. These three periods are understood as being driven by different forcing agents. The early 20th Century warming was caused by both increasing carbon dioxide and increasing solar activity. The mid-century cooling and stable period follow rapid industrialization and World War II, and the industrial activity following the war injected an abundance of particles into the atmosphere. These particles, called aerosol, reflect sunlight, which lead to a so-called "global dimming" which decreased the sunlight reaching the surface, thereby reducing the global temperature. The later warming, after 1980 or so is dominated by carbon dioxide, which became a stronger forcing agent than the dimming aerosol as air pollution became more strongly regulated while carbon dioxide continued to rise. (reference [])
The analysis of these global patterns of temperature change really brings out the fact that many factors contribute to the global average temperature. A sub-discipline of climatology has emerged that tries to quantify the contributions of various factors to observed climate change; this field is often put into a category called attribution studies. A recent example is Foster & Rahmstorf (2011), that attempts to remove natural factors from the late 20th-early 21st Century climate records. They are able to account for factors such as ENSO, volcanos, and solar variability, leaving the remainder of the trend explained by human-caused factors. Their conclusion is that nearly all of the observed temperature trend is associated with radiative forcing that is ultimately due to rising carbon dioxide concentrations that come from burning fossil fuel.
While the above examples show evidence for the temperature change in the 20th Century, recent climate change is expressed in numerous other climatic indicators. One example is the extent of Arctic sea-ice. As the climate has changed over the past few decades, there has been an unprecedented decline in the area covered by sea-ice in the Arctic Ocean, especially late in the warm season which is also the sea-ice melt season ( Kinnard et al. 2011 ). The most direct way available to track these changes uses satellite observations. The figure at the right shows observed Arctic sea-ice concentration (in percent) for September 1979 and September 2010 from a set of passive microwave observations [NSIDC] These two months show an example of the change in coverage of sea-ice in the Arctic Ocean: September 2010 shows substantially less ice than 1979. September is the end of the melt-season, so these maps are an illustration of the minimum sea-ice coverage in these years. The time series below the maps show the monthly mean sea-ice extent from late 1978 through 2010. Sea-ice extent is the area covered by sea-ice concentration of at least 15%. The annual cycle of sea-ice extent is quite large, oscillating between 15 and 6 million square kilometers in most years. Looking at a particular month shows the long-term trend more obviously. The blue line in the time series plot highlights September sea-ice extent, and it is clear that there is a decline in the September sea-ice minimum. Both the concentration and the extent are an expression of the area covered by ice, but at least as important is the volume of sea-ice. Observations show a pronounced decline in sea-ice volume, similar to the decline in sea-ice extent. The volume is decreasing in the winter months even more strongly than the extent is; this is because the ice that forms in the winter is thin and has less and less volume as the thick ice (that survives through the summer) disappears.
Another indicator of the changing climate is the change in global sea level that has been observed over the last several decades. There are two main effects that are important for the global mean sea-level change. First, as the near-surface temperature increases, heat flows into the ocean's surface waters. As the water temperature rises, the water slightly expands, similar to other materials. This direct temperature effect is called the steric effect on sea-level. The second major factor is the change in the mass balance of the oceans. As the climate has warmed, ice that has been sequestered on land as ice has begun to melt. Much of this melted ice has found its way to the sea, either directly as in the case of calving of ice from Greenland and Antarctic or by making its way to the ocean via streams and rivers. Recent estimates suggest that these two effects combine to account for a long-term trend in sea-level that is more than 3 mm of sea-level rise per year. 
Some of the observed evidence that Earth's climate is changing:
- Global Temperatures are Rising
- Melting Glaciers
- Sea Levels Rising
- Weather patterns are become more difficult to predict
- Seasonality of weather and temperatures are changing