The Earth's surface has a net negative electric charge, and this is largely due to a phenomenon known as the global electric circuit. The Earth's atmosphere, specifically the ionosphere and the surface of the Earth, forms a giant electric circuit. Let's explain how this works.
The Earth's surface and the ionosphere form the two "plates" of a giant capacitor. The ionosphere is positively charged, mainly due to the influence of solar radiation that ionizes the atoms and molecules in the upper atmosphere, causing them to gain a net positive charge. The Earth's surface, in turn, becomes negatively charged to maintain an overall electrical balance.
The separation of charges leads to an electric field between the surface of the Earth and the ionosphere. On a clear day, this electric field is about 100 Volts per meter above the Earth's surface.
The charges are constantly being redistributed and balanced due to various atmospheric phenomena. Thunderstorms play a critical role in maintaining this global electric circuit. Within a thunderstorm, there are updrafts and downdrafts that separate charge. The process isn't entirely understood, but it's thought that ice particles colliding within the storm cloud cause a separation of charge, with lighter, positively charged particles tending to move upwards and heavier, negatively charged particles tending to fall downwards.
The negatively charged particles near the bottom of the cloud can induce a positive charge on the Earth's surface below. If the charge separation becomes too great, it is resolved by a lightning strike. The lightning equalizes the charge by transferring negative charge from the cloud to the ground. This process is happening continuously in the thousands of thunderstorms occurring at any given moment around the world.
It's also worth noting that the Earth's surface can obtain a net negative charge from other natural processes such as the influx of cosmic rays and ultraviolet radiation.
However, it's important to understand that the net negative charge is very, very small in comparison to the size and mass of the Earth. Moreover, the distribution of charge is not uniform and it can vary with time and location due to variations in atmospheric conditions, solar activity, and other factors.