Swedish climate change activist Greta Thunberg has responded to the criticism she received in the wake of her viral speech at the UN summit. During the speech, she encouraged world leaders to tackle the global climate change emergency.

"People are suffering, people are dying, entire ecosystems are collapsing. We are in the beginning of a mass extinction and all you can talk about is money and fairy tales of eternal economic growth," Thunberg said.

This is the speech in full:

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The 16-year-old's words were subsequently met with criticism from some.

Now, the teenager has hit back at her "haters" on Twitter.

Greta Thunberg. [[imagecaption|| Credit: PA Images]]

"As you may have noticed, the haters are as active as ever - going after me, my looks, my clothes, my behaviour and my differences," Thunberg said.

She continued: "It seems they will cross every possible line to avert the focus, since they are so desperate not to talk about the climate and ecological crisis. Being different is not an illness and the current, best available science is not opinions - it's facts."

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Earlier this week, Fox News network issued a statement of apology after a guest described the 16-year-old as a "mentally ill Swedish child."

The statement read;

“The comment made by Michael Knowles who was a guest on “The Story” tonight was disgraceful — we apologize to Greta Thunberg and to our viewers.”

Thunberg wrote;

"I honestly don't understand why adults would choose to spend their time mocking and threatening teenagers and children for promoting science, when they could do something good instead. I guess they must simply feel so threatened by us."

The teen is pictured below glaring at President Trump during the climate change summit.

Greta Thunberg scowling at Donald Trump. [[imagecaption|| Credit: PA Images]]

Examining the backlash, Buzzfeed reported that the abuse Thunberg receiving is perhaps, regrettably, to be expected. Typically, young activists, particularly those who are female, are subject to a slew of abuse which can include death threats and racist messages.

Greta Thunberg responds to 'haters', asks why adults 'spend their time mocking and threatening teenagers for promoting science'