Theory
The basic hypothesis is: global warming increases the temperature also inside the planet, this will cause an increase in the frequency and intensity of volcanic eruptions and earthquakes.
Planet Earth is an active thermodynamic system that creates heat - energy inside and dissipates it towards outer space; the amount of heat - energy transmitted depends on the characteristics of the material that divides them, in our case from: outer core, mantle and, as far as my theory is concerned, I must emphasize the importance that the Earth's crust, oceans and atmosphere have in the equilibrium of this thermal system.

Global warming increases the average temperature of the soil, water and atmosphere, altering the equilibrium because it creates a “blanket” effect (the blanket we put on the bed during the winter is used for this, to retain the heat produced by our body) this effect decreases the amount of heat that the planet loses into space.
Moreover, if we consider the fact that the core has a fixed temperature of about 5000 degrees, this energy which cannot exit the planet will tends to accumulate, altering the thermal equilibrium between the planet and space.
This energy which is no longer dissipated remains stored mostly in the mantle because it is the area closer to the heat source; this makes the rock more fluid and the convective movements more intense. Both things have the effect of eroding the Earth's crust from below and the tectonic plates that floating above the mantle will become increasingly thinner, lighter and easier to move. All this will lead to only one thing: increase in faults. Eruptions, earthquakes.

In 1970, drilling in the ground began in Kola, Russia with the aim of going as far as possible and a record depth of approximately 12 km was reached. While the drill was going down it was realized that the mechanical characteristics of the rock were changing; if we combine this with the consideration that in many points the Earth's crust has a thickness.
Furthermore, warming affects the entire globe at the same time. We will no longer have a single earthquake or a single volcano everywhere, but we will have more events ever more intense and frequent; the effects would be added, especially the ash dispersed in the atmosphere.
If there were an increase in global eruptions, if 4 or 5 volcanoes like Pinatubo, Cracatoa or other unpronouncable Icelandic volcanoes erupt simultaneously, the tons of gas and ash emitted into the atmosphere could lower the global temperature, to the point that a new ice age was created. ?

Evidence
If the hypotheses are founded, there should be a general increase in all geological events generated by phenomena occurring within the planet, both in number and intensity, distributed simultaneously throughout the globe.

Volcanoes and faults
Deformations of volcanic cones; greater gas emission; sudden awakening of dormant volcanoes; perhaps, because the equatorial band is warmer, a greater number of events should be found along it than in the polar zones; the increase in the frequency of low-intensity seismic swarms could indicate an increase in the speed of movement.

Geisers and fumaroles
Any changes in intensity and chemical composition of gases should be easier to detect and study because they are low energy phenomena and therefore more sensitive to small changes in temperature (less energy is needed to activate a geiser than a volcano or to move a plate).

Hot spot activity
Kilawea has been erupting constantly for many years and its lava appears to be getting more and more liquid; this could indicate that there is much more energy deep down or that the origin of the magma is closer to the surface, so it has less time to cool before coming out. Yellowstone should also have been “inflating” at an increasing rate.

Changes in the chemical composition of thermal waters
An increasing concentration of minerals or the possible presence of increasingly heavier elements could indicate that they have been pushed by a more intense force than in the past or that they come from a greater depth.

Continental drift
The speed of plate movement should be increased for two reasons: the energy retained by the mantle increases the power of convective movements, as a result, the plates floating above will be moved with greater force; secondly, the molten rock zone has the ability to erode the plates from below, thus making them lighter and even easier to move.

Caves and mines
Being closer to the mantle, the temperature should also be significantly increased inside these places. This makes them particularly interesting because, perhaps, thanks to any recordings of temperatures over the years, it is possible to directly measure the effect that global warming has on the temperature of the underground.

Geothermal systems
Perhaps, working with hot water coming from the underground, these systems could have recorded any increases in water temperature or pressure in the pipes. These systems could be important to monitoring the advance of heat coming from the underground: water is sensitive to even small variations and measuring temperature and pressure should already be part of the normal operation of these systems.

Further thoughts:

• Ice ages
Considering the planet as an active thermodynamic system with self-regulating mechanisms could explain why ice ages appear to have occurred in a regular time cycle.

• Sun
It would be interesting to study the Sun as an active thermodynamic system, equipped with self-regulating mechanisms that stabilize its activity. It seems that regularly every 11 years a cycle of spots is created on its surface.

• Ocean floor
The increase of water in the oceans creates an increase in the weight exerted on the floor and pushes the water deeper into the rocks, decreasing their compactness; this could change the mechanical characteristics of the sea floor and create deformations along the subduction lines of the plates. The boundary lines between oceanic and continental plates, where one begins to slide under the other (as in South America), are on the same plane as the rest of the sea bed and the thrust exercising is in axis with it; behind them (moving away from the continent) due to the greater weight of the water, the sea bed will tendency to lower and consequently the plane along which it exerts pressure will no longer be aligned with the continental plate. Instead of having a flat ocean floor that suddenly sinks under the continents, the lowering of the slide plane could create a fold, a hill, a band of rock trapped between the margin, now blocked under the continent, and the advancing ocean floor. Sort of like paper that gets jammed in a printer. In the short term, the shift of the thrust plane should limit the occurrences of earthquakes, but in the long term, the continuous accumulation of kinetic energy could generate earthquakes of a magnitude never recorded before. Moreover, considering the decreased compactness of the rocks due to the infiltrations of water ever deeper, the less compact ocean floor may no longer be able to withstand the pressure it receives from the ocean trenches that push it under the continent, it could break before going under the plate and reverse its direction; in some areas we could witness a reversal of the direction of subduction: the sea floor, instead of sinking under the continent, begins to move upwards, overlapping the continent downwards. Some recent research seem to indicate that the drivers of subduction are not the crust fractures in the ocean trenches which continuously produce new magma and push away the old one under the continent, but the opposite: it is the sea bed itself that drags everything else with it as it sinks; it is the edge of the plate that pulls and tears the center of the ocean, creating the ridge. Due to the changed mechanical capacities of the rock, the sea bed may no longer be able to hold the traction and may fall at any point, creating “parallel ridges”.

• Thermal equilibrium band
This band, which covers the entire globe, is located in the depth of the Earth, is the place where the heat generated by the core and that derived from the action of the Sun are equal (in practice, if you go deep the temperature increases due to the core, if instead you get closer to the surface the increase is due to the Sun); here the thermodynamic system Earth - space finds the equilibrium between the quantity of heat produced internally and that produced externally by the Sun. The idea of a thermal equilibrium belts could help explain the shape of the Earth. At both poles the almost absence of solar heat causes internal energy to be dissipated more easily and more depthwide; the rock is colder, more compact and heavier, weighing towards the core with a greater force than that exerted by rocks found along the equator. To what has just been said, we must obviously add the physical effect of dislocation, the outward drift due to the centrifugal force; in my opinion this does not conflict with my hypotheses, because the rock along the equator's axis is warmer, therefore more fluid than that of the poles and this facilitates the movement of the rocks outwards, along the plane of the equator. Consistently with this hypothesis, it can be noted that the planet does not have an elliptical shape and should appear flasked at the South Pole, as the weight of the continental rock is certainly greater than that of the water and the ice flee at the North Pole. In support of this hypothesis, it can also be noted that Mars has the most spherical shape. This is because it does not produce the energy necessary to melting the rock and move it; the rocks in the “crust and mantle” bands remain stable and compact, creating a homogeneous pressure towards the center, from the poles to the equator. The rocks are subject to a single force, gravity, which gives objects their typical spherical shape.