Here is a good way to think about the effect of the oceans. Suppose we have a sealed glass cylinder containing equal volumes of air and water. If it is just sitting at rest with no energy going in or out through the walls of the container, the air and water will settle down to the same temperature. Now heat the bottom of the cylinder for a few minutes and turn off the heater. After a while, once again the temperature of the air and water will be equal, but both will be warmer, say by 1 degree. But because the water is nearly 1,000 times as dense as air and can absorb a little more than 4 times the amount of heat per unit mass, the proportion of energy from the heater that went into warming the water was about 4,000 times more than the amount of energy used to heat the air.
Now let’s do a second experiment. This time, add enough black dye to the water to make it opaque and shine a powerful flashlight down through the glass top of the cylinder. The light passes through the air but is absorbed at the very top of the water, heating it. So the top of the water warms up, and since that is the part that is in contact with the air, the air warms up too. But the water below the surface is not heated by the light, which never makes it down below the surface, so it remains at the temperature it had before. But slowly – very slowly—the warmth of the surface water is diffused down into the deep water and this both warms the deep water and cools the surface water and with it, the air.
Thus after we turn on the flashlight there will be an initial fast warming of the air and surface water, followed by a very slow increase in the temperature of the whole system. Eventually, the water and air will reach a new, warmer temperature. How long it takes to do so will depend on how rapidly heat diffuses downward into the deep water.
By analogy, we could account for the lag between heat input and temperature change in the real world if we had a simple theory for how heat penetrates the ocean depths. We know that heat is mixed rapidly downward to a depth of between 20 and 150 meters (60 and 150 feet), depending on location and time of year. If heat did not penetrate deeper, then the 20–150 meter penetration would give a lag of around two years, which would be hardly noticeable in Figure 2. But we know from measurements that heat manages to circulate much deeper in the ocean, taking quite a long time to do so. Just how this happens is complex, and for this and other reasons we turn to comprehensive climate models, about which more in due course.