This point is in a way (almost unbelievably) actually correct, although not in the way that the it is put in the article because it is the change in the Earth's rotational speed that requires the subtraction of a leap second, not a change in "the essence of time".
Nevertheless, time really is being impacted by global warming.
We all tend to assume that time passes at the same rate everywhere on Earth, but this is not the case. According to Einstein's Theory of General Relativity (no, seriously!) time does not flow at a constant rate everywhere in the Universe, but will flow at different rates depending on the warpage of space-time by gravity, with time passing more slowly as gravity increases. Global warming is moving mass from the poles to the oceans and is therefore impacting gravity at different points on the Earth's surface (now reduced at the poles and increased in the oceans - mostly at or near the equator) and with it the flow rate of time at those different points on the Earth's surface. So as the polar ice melts time passes more quickly at the poles and more slowly at the equator. The difference is however incredibly miniscule.