Emissions scenarios , estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological , technological , and natural developments.  In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced.   Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century.  Emission scenarios, combined with modelling of the carbon cycle , have been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future. Using the six IPCC SRES "marker" scenarios, models suggest that by the year 2100, the atmospheric concentration of CO 2 could range between 541 and 970 ppm.  This is 90–250% above the concentration in the year 1750.
When it started in 2007, the GCCA was working in just four pilot countries. Today, it supports 51 programmes in 38 countries , 8 regions and subregions and at the global level. In 2014, the GCCA, entered a new phase through its flagship initiative - the GCCA+.
All images on this site lovingly provided by our visual content partner
Solar output also varies on shorter time scales, including the 11-year solar cycle  and longer-term modulations .  Solar intensity variations possibly as a result of the Wolf, Spörer and Maunder Minimum are considered to have been influential in triggering the Little Ice Age,  and some of the warming observed from 1900 to 1950. The cyclical nature of the Sun's energy output is not yet fully understood; it differs from the very slow change that is happening within the Sun as it ages and evolves. Research indicates that solar variability has had effects including the Maunder minimum from 1645 to 1715 . , part of the Little Ice Age from 1550 to 1850 . that was marked by relative cooling and greater glacier extent than the centuries before and afterward.   Some studies point toward solar radiation increases from cyclical sunspot activity affecting global warming, and climate may be influenced by the sum of all effects (solar variation, anthropogenic radiative forcings , etc.).  
IPCC: Global average sea level in the last interglacial (Eemian) period
(130,000-111,000 years ago) was likely 13 to 20 feet (4 to 6 meters) higher
than during the 20th century, mainly due to the retreat of polar ice. Ice
core data indicate that average Arctic temperatures at that time were
to deg. F (3 to 5 deg. C) higher than present, because of differences
in the Earth’s orbit. The Greenland ice sheet and other Arctic ice fields
likely contributed no more than 13 feet (4 meters) of the observed sea
level rise. There may also have
been a contribution from Antarctica. Note in the chart how the rate of
sea level rise is very low compared to that when the ice age wanes. ( IPCC
NOAA: The Mid-Cretaceous period is one period in the geologic past that stands out as distinctly warmer than today, particularly at high latitudes. During the mid-Cretaceous Period, 120-90 million years ago, fossil remains of plants and animals believed to inhabit warm environments, were found at much higher latitudes. Breadfruit trees apparently grew as far north as Greenland (55° N), and in the oceans, warm water corals grew farther away from the equator in both hemispheres....... The mid-Cretaceous was characterized by geography and an ocean circulation that was vastly different from today; as well as higher carbon dioxide levels (at least 2 to 4 times higher than today). This indicates that the mid-Cretaceous climate system was different from that of today or any we might have in the future. Explanations evoking ocean and atmospheric circulation patterns radically different from today have been proposed to explain the climate of the mid-Cretaceous; however, there is no scientific consensus on how the Mid-Cretaceous warm climate came about (source: NOAA Paleo Climatology program) . In some ancient times when CO2 levels were very high, ocean organisms with shells based on silica replaced those with shells based on calcium.