Certain planetary phenomenon will leverage the warming caused by mankind and bring about greater climatic effects in the long term. Here are a few:

Cyclones, Trees and Carbon

The CSIRO predicts more damaging cyclones across the north of Australia and that they will damage more forest in their path. This is especially relevant to larger and older trees that are less able to weather a cyclone. The damaged limbs and dead trees fall to the ground to rot and give up the carbon locked away in them. The larger trees will not be replaced as the cyclones prune them from the forests reducing the average age of the forest trees and consequently the size of the trees and the amount of carbon they store. One estimate has the amount of carbon released as equivalent to a years worth of burning fossil fuels..
Source: CSIRO

Southern Ocean Carbon Sink

Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change
The first evidence that recent climate change has weakened one the Earth's natural carbon 'sinks' that takes up about half the human related carbon dieoxide emissions has been published in the journal - Science Full story: Science Journal

“The oceans currently act as the world's main reservoir of carbon dioxide, and at present, the small creatures on the surface waters that take up the carbon die, and then sink to the ooze on the sea floor where their carbon is locked up for thousands or millions of years.
As the oceans become more acidic, however, this process is expected to break down, saturating the upper layers with calcium carbonate and reducing their ability to absorb CO2 from the air. At the same time there will be changes to the ocean's physical ability to buffer the rising acidity, while warmer conditions will also inhibit the dissolution of CO2.
Together these factors mean that the seas ability to soak up excess CO2 from the air may decline. This in turn will lead to a further rise in atmospheric carbon levels which, scientists fear, could trigger a runaway greenhouse effect. Today's international scientific predictions of global warming rates of a few degrees that have received prominence in the media do not yet factor in this possibility. It remains a sleeper dynamic that could cook the planet far more rapidly and to high temperatures. How hot and how quickly remains uncertain.”
Source: ECOS magazine, CSIRO Publishing April-May 2008 page 18 ‘Acid Oceans’
More information: CoEcrs: www.coralcoe.org.au
Veron JEN (2008) A Reef in Time: the Great Barrier Reef from Beginning to End. Belknap Press USA. http://www.amazon.com/Reef-Time-Great-Barrier-Beginning/dp/0674026799

Also, warmer oceans hold less CO2 so as the oceans warm their capacity to act as a carbon sink will be further reduced.

Siberian Bog Methane

Thawing Siberian bogs are releasing more of the greenhouse gas methane than previously believed, according to new scientific research.
Scientists from Russia and the US measured methane bubbling from a number of thawing lakes.
Writing in the journal Nature, they suggest the methane release is hastened by warmer temperatures, positively feeding back into global warming.
Methane's contribution to present-day global warming is second only to CO2.
Full story: http://news.bbc.co.uk/2/hi/science/nature/5321046.stm

Amazon Basin

The Amazon Basin holds a vast amount of carbon (about 70 billion tonnes) in it's forest and soils. If the rainforest is cleared or 'collapses' due to heat stress that carbon will enter the atmosphere, accelerating the rate of climate change.
Full story: http://www.nasa.gov/centers/goddard/news/topstory/2006/amazon_crops.html

Climate Lag

The oceans posses huge thermal inertia, that is they take a long while to heat up. The delay between atmospheric heat transferring to ocean heat is about twenty to thirty years. This means that even if we were able to stop the increase in greenhouse gasses the oceans would continue to heat up for another two to three decades. This heat would expand the water in the oceans causing sea level rises and further heat the planet.

Global Dimming

Once the air is clear of pollutants more sunlight will penetrate the atmosphere to reach earth, heating it further. The heat trapped by the greenhouse gasses will further contribute to global warming.

Contrails

Contrails are the linear white clouds that form in the jetstream of aircraft. As they are bright white they act to reflect incoming sunlight and so have a mild cooling effect on the climate during the day and can trap heat in overnight. As people fly less (due to rising cost of fuel and the pricing of carbon emissions into air travel) there will be less contrails and therefore a greater variation between day and night temperatures. Scientists are divided but it seems that less contrails will likely make global warming worse. The effect of contrails was documented during three days in September 2001 when all of the United States aircraft were grounded.
Full story: http://www.abc.net.au/science/news/stories/s643595.htm

Phytoplankton

Some researchers believe that if the surface ocean temperatures are allowed to rise by 6° phytoplankton will not be able to survive in our oceans. (That is if they have not already died from ocean as certification, see above) This will reduce the ability of the oceans to soak up excess carbon dioxide to almost zero. In fact the oceans may well become a source of greenhouse gases, as the phytoplankton diet en masse and decompose at the bottom of the seas. The role of phytoplankton in adding oxygen to our atmosphere would stop and the process will could potentially add another 2° of warming to the earth.
Source: Climate Code Red http://www.climatecodered.net/

Feedback loops as the Planet Warms...

With the warming of the planet several other mechanisms come into play that will exaggerate warming further still, such as;

Warmer oceans hold less carbon dieoxide
The oceans have been absorbing about half of the CO2 released by man but as they warm their ability to absorb CO2 is reduced.

A warmer atmosphere holds more moisture
Although not regulated as an emission water vapour is a major greenhouse gas. As the atmosphere warms more moisture will be held in the atmosphere further enhancing the greenhouse effect.

Less iron fertilisation of oceans
A warmer, wetter climate may mean less iron-rich dust blown onto the oceans. Iron promotes the growth of plankton (a plant) which draw CO2 from the atmosphere and when they die often sink to the bottom locking up the CO2 in storage at the bottom of the sea.

Algae and cloud formation
Algae release a chemical called dimethyl sulphide (DMS) into the atmosphere that is connected with cloud formation. Warmer oceans mean less algae and less bright white clouds which reflect the suns rays. This means more sunlight hits oceans and land, warming the planet further. (Lovelock and Kump 1994 and Lovelock 2006)

Other 'Positive feedback loops' will also act to exacerbate global warming and climate change.