Coastal aquifers are under permanent threat of seawater intrusion (SI), the process of seawater driven landward at the base of the aquifers by variable density flow. Density contrast between fresh and saline waters, however, does not solely depend on salinity difference as considered in most SI studies but also their temperatures. Based on survey data, we show that temperature variation of up to 15°C is fairly common between seawater and groundwater in various parts of the global coastline and the difference could be either expanded or shrunk under the warming climate depending on their current situation and their location in the world. Therefore, temperature effects should not be readily neglected. Furthermore, temperature and salinity gradients coexisting alongside each other suggests interesting changes of flow patterns and circulation following double diffusive mechanism. Using results from laboratory experiments and numerical simulations, we figure out that the intrusion process enhances with colder seawater and reduces with warmer seawater. More importantly, pore-water flow in the saltwater wedge was modified significantly with a second circulation cell formed near seaward boundary and the regular circulation cell is squeezed into a smaller area with stronger inflow from the beach surface. The thermal impact and coastal vulnerability were then calculated for all coastal areas around the world based on their current temperature contrast and projected future change. The results reveal that a large portion of world coastline is thermally sensitive and vulnerable to changes of land surface and ocean temperature in the warming climate.