If that's the case, shouldn't we investigate that whatever caused the rise in the past isn't responsible for causing at least some of it today? Or can we definitely tell it was due to solar activity ?
We have.
It isn't.
Honestly, why do you assume climate scientists are idiots? The people telling you that humans are causing global warming are the same people who've spent the most time and effort determining how and why temperature has varied in the past.
Ignore the trend evidence, just look at the CO
2. We know, for a fact, that humans are releasing around 7 thousand million tonnes of C in the form of CO
2 (~24 thousand million tonnes of CO
2) into the atmosphere each year and that total levels of CO
2 are rising by around 2 thousand million tonnes of CO
2 each year. We know, for a fact, that CO
2 absorbs infrared radiation because it transfers into vibrational energy in the bonds between the O atoms and the central C atom. We know that the earth's surface radiates 1.98 x 10
17 W of energy as infrared energy each year and that the amount of that energy that escapes into space is a vital determining factor of surface temperature and we know that CO
2 is the second most important greenhouse gas (meaning a gas that absorbs IR radiation and thus traps heat energy in the atmosphere) in the atmosphere. The most important is H
2O but it's quantity is determined primarily by atmospheric temperature and thus follows the level of CO
2 in the atmosphere.
Given these facts what possible way is there for human generated CO
2 not causing global warming? What do you think is happening to the additional trapped energy if we're not?