Climate change cannot be “turned off” immediately.  The concentrations of greenhouse gases in the atmosphere and heat stored in the oceans, rather than current emissions, are what determine how warm the climate will be.  To draw upon a water resource analogy, atmospheric concentrations of CO₂ can be thought of as a large reservoir, while current emissions are like a small stream entering the reservoir. The level of the reservoir rises or falls depending upon whether the natural draw-down processes (e.g. evaporation in the case of water) are larger or smaller than the current rate of inflow.  There are natural processes by which CO₂ and other greenhouse gases are removed from the atmosphere (including uptake and storage of CO₂ in biota, soils, oceans and ocean sediments), but we are currently adding these gases much faster than they are being removed.  In addition, these natural sinks may become saturated as CO₂ concentrations rise.  On average, natural sinks currently remove over half of the carbon emitted by fossil fuel use each year, but these processes almost certainly will become less effective in a warmer world.  For example, the solubility of CO₂ in seawater declines as the water warms (IGBP 2001), which would reduce the effectiveness of the ocean sink.

It is also important to understand that the thermal inertia of the oceans results in a time lag between changes in atmospheric greenhouse gas concentrations and changes in global average temperatures.  Wigley (2005) estimates that climate would continue to warm and sea levels would continue to rise for several centuries, even if greenhouse gas concentrations were immediately stabilized.  In other words, stopping climate change would require stabilizing atmospheric concentrations of greenhouse gases,¹ and then waiting for a long time while the climate system gradually equilibrated.   The reduction in emissions that would be required to do that depends on the levels at which CO₂ and other greenhouse gas concentrations are to be stabilized, and the target date for that stabilization. 

1Note that it is actually the radiative forcing of the entire suite of greenhouse gases that would have to be stabilized, so there may be several ways to achieve a mix of emission reductions to meet any specific stabilization target.