The term footprint has come to mean the balance of carbon dioxide (CO2) taken out of the atmosphere and carbon dioxide put into the atmosphere. CO2 is removed from the atmosphere primarily by photosynthesis and enters the atmosphere again either through respiration or combustion. It is believed that CO2 plays a direct role in the thermal balance in the atmosphere, but we should not overlook the effect of water vapor on the atmosphere, as it is also a product of respiration, and transpiration which usually occurs in conjunction with photosynthesis in terrestrial biomes.
The data that is most references, and has been a standard touchstone in oceanographic and atmospheric textbooks comes from a National Oceanographic and Atmospheric Administration (NOAA) run field station in Mauna Loa, Hawaii. Hawaii has been chosen as a field station for observation of global trends in the atmosphere because it is so far away from large scale local perturbations to the atmosphere by human activity. In other words, it is presumed that the atmosphere has been well mixed by the time it reaches an observation point in Hawaii, and so only global trends are observed. NOAA keeps recent data online here, and you should look at it to follow the rest of the discussion. The site is also listed in reference  .
There are two things that we immediately recognize from the month by month plot of CO2 at Mauna Loa, here after called the Mauna Loa plot. First is that the amount of CO2 in the atmosphere is continually increasing, as emphasized by the slope of the plot of average values. The second thing that we notice is that there is a yearly cycle to be observed in the data. It is from the yearly cycle observed in the data that we can make inferences about the impact of different hemispheres on photosynthesis, and as a result, the CO2 balance in the atmosphere.
The time axis is broken down into quarters, and what we notice is that CO2 concentration reaches a maximum at the end of the first quarter, which corresponds to the end of winter in the northern hemisphere, and as such, the beginning of the spring bloom, or return of primary production, as the spring turns into summer, we see the negative slope of the CO2 concentration reaches a maximum at about the end of the second quarter, or the early summer peak in photo synthesis in the northern hemisphere.
Obviously then the reverse process begins, in the fall as the northern hemisphere cools and photosyntesis ends for the season. What this plot shows is there is a definite difference between the terrestrial balance of CO2 and the oceanic balance of CO2. If the ocean were as productive as the continents in terms of photosynthesis, then we would not expect there to be a cycle in the data. Even though biologists and ecologists consider many of the southern seas to be highly productive, in relation to the northern hemisphere they are not. The Mauna Loa data and trends are not new, they were in oceanographic texts in the 1980's when I first was introduced to them.
Other alarming trends have been monitored at least since the 1980's. Probably the most significant, at least as observed from satellite as a global scale event is the expansion of the Sahara desert and the continued desertification of North Africa. As a result of this trend, there was a recent writing topic sponsored on Lake Chad in the wikipedia, the answer is that the ever expanding Sahara is encroaching and swallowing the lake.
What happens in Northern Africa is by no means local to that continent, or hemisphere. The United States Geological Service ( USGS ) has recently ventured to discover why corals in the Caribbean and Florida are bleaching. Bleaching occurs when coral reef building organisms expel the pigmented symbiotic algae from which they derive food. The term bleaching is used because without the presence of pigments in algae, the corals appear white. One hypothesis presented is that dust originating in North Africa is part of the problem . Most scientist in the field believe that there are actually many factors that lead to coral bleaching, and in fact bleaching is a global phenomenon.
None the less, the dominant atmospheric "cargo" that "should" be emanating from North Africa is water vapor. If water vapor were being transported over the Atlantic instead of dust, A number of atmospheric characteristics would be different. There would be a greater take up of CO2 in the summer photosynthesis event. Cloud cover would reflect more of the suns energy into space and prevent Atlantic water from heating. Cooler Ocean water temperatures would give rise to fewer and milder tropical storms and hurricanes. There would be greater precipitation in Florida and the Atlantic coast, and temperatures would be milder because condensing precipitation has a tendency to release energy, and thus maintain a warmer temperature.
It is probably not a coincidence that Florida is now ( 2010 ) experiencing one of the COLDEST winters ever, and that may be attributable in part to less humidity coming off of the African continent due to desertification.
Loss of dense ground cover is probably a phenomenon characteristic of the entire Northern hemisphere, and Southern as well ( Brazil and Australia are probably both culprits and victims) . But even if the tide were to be turned, the growing disaster in Africa is of a scale that will be of global proportions and felt around the world, including Florida and the U.S.
 NOAA, Earth System Research Laboratory, Trends in Atmospheric Carbon Dioxide Retreived from here
 USGS, Coral Mortality and African Dust Regrieved from here