Very broadly speaking, you can decrease emissions (the regulated criteria/toxic kind, as in Euro-4) from a given engine by increasing the efficiency of the combustion in the cylinders, using cleaner fuel, by aftertreatments (catalysts, filters, etc.), or any combination of the three.

CO2 is a natural outcome of the combustion reaction. Combust the same amount of fuel more completely, and you’ll end up with more CO2.

Aftertreatment devices can also add to CO2. Oxidation catalysts, for example, convert the combustion byproducts of carbon monoxide (CO) and unburned hydrocarbons (HC) to CO2 and water.

That sounds like a bad deal, but it’s chemistry. The ideal (assuming you need to use a hydrocarbon-fueled combustion engine) would be to have a more efficient engine that allows you to downsize, thereby using less fuel. In other words, the slight increase in CO2 would be more than offset by the overall decrease in fuel consumption in a downsized, or just more efficient, engine.

In this particular case, though, it’s basically the same engine, but managed for better combustion.

You’ll note that there is also a fuel penalty. It’s a cleaner car than the pre-Euro-4 version, but it does burn slightly more fuel and produce more carbon dixoide.

The issue goes beyond this particular car. In a 1998 report, for example, Japan’s Transport Engineering Council recommended an emission cut to one-sixth (gasoline vehicles) and to one-third (diesel vehicles) of the then current levels. At the same time, the Council predicted that such regulations would decrease fuel economy by 5% for diesels, 3% for conventional gasoline engines and 6% for direct injection gasoline engines—all having an adverse effect on CO2 emissions. (Burn more fuel, generate more CO2.)