How are we sure that this isn't natural?

Yes, very sure.

Objects don't spontaneously rise in temperature. There are only a handful of physical explanations for why the Earth could warm ...

1. The orbit of our planet has brought us closer to the sun. There is no evidence our planet's orbit has changed by a sufficient amount to cause a 1°C rise in a century. In fact, the Earth is gradually moving away from the sun by about 15 cm each year. If we can measure that rate of change, we'd know by now that the warming was due to orbital variations.

2. The albedo of the planet has decreased, reflecting less sunlight back into space and therefore causing warming. In fact, we know the albedo has increased due to human activity and our clearing of forests for crops and our construction of concrete buildings. So we should be seeing the planet cool not warm. Volcanic activity, producing ash that lowers albedo might do it, but we've seen no evidence of a change in activity sufficient enough to cause warming. We're now measuring albedo with satellites and can see rises in temperature that aren't correlated with any change in albedo.

3. Solar output has increased. This is the natural explanation favoured by sceptics, but it's also the least probable. Solar output does vary but only by a fraction of 1%. We can correlate historical sunspot number counts to solar output, add in our more recent ground and satellite data, and determine the impact ... varations of about 0.2 °C. There's no evidence solar activity is the predominant driver of the warming we've seen. In fact, even as solar activity dropped, warming continued.

4. An increase in greenhouse gas concentration. The average temperature of the Earth is the point at which the rate of radiant heat transfer from the Earth into space matches the radiant heat transfer from the sun to the Earth. Increasing the concentration of greenhouse gases results in a lowering of the heat transfer from the Earth to space since they absorb IR photons and emit them in random directions (including back towards the Earth) and so the temperature of the Earth must increase until that balance is restored.

We've seen an increase in CO2 concentrations from 300 to 400 ppm in about 120 years. When we look at bubbles of air in ice cores, we find that the 100 ppm rise in 120 years is faster than anything that's occured in about the last million years. We know that CO2 rise is caused by us ... we can look at carbon isotope concentrations (carbon naturally circulating in the environment has a different ratio to carbon in fossil fuels buried underground for millions of years) and we can do simple accounting (we know how much fossil fuels we buy and burn each year). We are causing the CO2 rise.

Based on the rate of increase of CO2, the calculated radiative forcing was determined to be 0.2 Watts per square metre of Earth's surface per decade. The rate measured by Berkeley scientists in both the middle of the US and Alaska over 11 years (by measuring the increase in energy radiated towards the ground in CO2 emission bands) was 0.2 W/m^2/decade.

In other words, we have direct experimental evidence that our CO2 emissions are having exactly the effect the laws of physics told us it would. The question is no longer whether global warming exists, or whether we are causing it. The question is what, if anything, we do about it!

Daro says the temperature has been "consistent" for the last 10,000 years, then posts a chart where you can't even see where we're at today. Here's one that does. See the 2016 arrow? 

Anyway your question can be answered in several ways. Scientists overwhelmingly agree that the current rise in temperatures is due to an increase in greenhouse gases in the atmosphere. This has been studied for decades, and this is the conclusion that has been reached by thousands of experts based on the evidence. It's not some knee-jerk on the part of radical environmentalists. 

Scientists overwhelmingly agree that the current rise in temperatures we are seeing is due to an increase in greenhouse gases in the atmosphere. 

--This is based on real-world observations: the warming has certain signatures that make it unique to a warming caused by an enhanced greenhouse effect, such as a cooling upper atmosphere (the lower atmosphere near the surface is warming), and a declining diurnal temperature range. 

--Add to that the fact that the swing is much larger and more global in extent, and happening much more quickly, than anything we see in the geological record. 

--Add to that the fact that all known natural causes of warming have been ruled out. 

--Add to that the fact that we understand really well how greenhouse gases absorb infrared radiation, and can calculate based on how much we know they have increased, how much warming we should expect to have seen -- and that these calculations match observations very well. 

--Add to that the fact that we have literally measured the infrared radiation increases to the surface due to the extra greenhouse gases in the atmosphere, both from space and with ground-based instruments. 

So of course it's a good question -- it's a critical question -- that's why it was asked a long time ago and answered very thoroughly. Happy to point you to direct links to scientific papers for any of the above points if you like. 

The recent cycles have been very consistent for the last 10,000 years.

We are currently in a period that real scientists call the "inter-glacial" a mild warm period between massive ice ages. 20,000 years ago (as the chart shows) the shift began and has been incredibly stable during the entire time that man and civilization has developed.

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As one can see the Earth is cooling, not warming. Doesnt happen in a straight line as idiots wud have us believe.

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Most of the last 500,000,000 years have been far warmer than what the government is trying to scare us with.

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Note: The real Dirac (now dead) may have been an actual physicist but Yahoos fake Dirac is just some kid in his mamas basement breathing too much CO2 for his brain to operate properly. He has proven that many times over. Always has to resort to reporting and removing anything that debunques his drivel, using multi-sock muppet accounts.

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Response to fake Dirac muppet:

The chart i show not only shows today but gives the projections for the next 50 years or so. It goes straight up for the future even tho the government flunkies have never, ever been right in their future scaremongering. They keep having to scramble to explain why American made Global Warming keeps resulting in massive cold spells.

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Rising levels of airborne carbon dioxide don’t cause global temperatures to rise…. temperature changed first and some 700 years later a change in aerial content of carbon dioxide followed.

~Dr. Lucka Kajfež Bogataj, Nobel Peace Prize winning climatologist

Much more progress is necessary regarding our current understanding of climate and our abilities to model it.

~Dr Oliver Frauenfeld

I never fully accepted or denied the anthropogenic global warming concept until the furore started after NASA’s James Hansen’s wild claims in the late 1980s. I went to the [scientific] literature to study the basis of the claim, starting with first principles. My studies then led me to believe that the claims were false.

~Dr Lee Gerhard

I am positively convinced that the anthropogenic global warming theory is wrong.

~Prominent Hungarian Physicist Dr. Miklós Zágoni