Besides the ecological footprint you might also have heard of the carbon footprint and the water footprint. Only carbon footprint makes sense; I criticized ecological footprint in an earlier post, but I did not mention another problem that also holds for water footprint.
Why do I think carbon footprint makes sense? It's about one environmental problem: climate change. It's about one type of pollutant, greenhouse gases such as carbon dioxide and methane, and it doesn't matter where you emit them. Whether you burn coal in Las Vegas, Shanghai, or Maasbree: the radiative forcing of the greenhouse gases you emit is similar.
How different that is with water. Growing fairly thirsty crops in Las Vegas is a much bigger problem than growing very thirsty crops in Maasbree, simply because there is much more water in the Dutch province of Limburg than in the American state of Nevada. In Nevada you would have to extract groundwater to irrigate your crops, and you would have to extract a lot of it because a big part of what you spray on your crops will evaporate before your crops can drink it. The groundwater aquifers you extract your water from will take a very long time to be replenished. In Limburg, on the other hand, you can use water from the Meuse (which is what Maasbree is named after), and even if you extract groundwater there is plenty of rain and river water to replenish it. Nevertheless, the water footprint will simply look at water use or the thirstiness of your crops, so growing very thirsty crops in areas where there is plenty of water is worse than growing somewhat less thirsty crops in areas where water is very scarce. Water depletion is a local problem: you can't compare Dutch water to Nevadan water. This problem does not hold for greenhouse gas because a Dutch ton of CO2 has the same effect as a Nevadan ton of CO2.
Ecological footprint has the same problem. Here the unit is not tons of carbon or liters of water, but hectares of land. With land it is important to consider the opportunity cost: if, say, you would use it to store carbon, what uses of the land would you have to forgo? In other words: if you would not use it to store carbon, what else would you have done with it? And then it turns out that it actually matters a lot whether we are talking about highly productive land in a well-drained and fertile Dutch polder or some remote wasteland with no biodiversity to speak of. Ecological footprint aggregates all land used by a person, a city, or a product, including the hypothetical hectares one would need to store carbon. Whether it is office space in the centre of Hong Kong, a highly diverse rainforest in Brazil, the fertile polder of Het Groene Hart or the barren desert of the Sahara: land is land, according to the ecological footprint. But it isn't, just like water is not water.