Many reasons for planetary emergency
Too late to avoid going far above 2°C. Leading world climate change science advisor warns climate target 'out the window 'BBC News 23 Aug 2012. Professor Sir Robert Watson said that the hope of restricting the average temperature rise to 2C was "out the window". He said that the likely rise is 3 to 4C and may be as high as 5C - with dire consequences. He presented the situation to the 2012 American Geophysical Union conference. With an radical drastic emergency response it is theoretically possible to keep under 2C but Robert Watson's prediction was based on the reality there will be no such response because the national pledges made are so paltry.

By definition any risk ​​of not stabilizing below 2C is a climate planetary emergency.
Arctic planetary emergency
o Snow and sea ice ​melt
o Methane feedback emissions​
o Runaway global heating​
Commitment / 'locking in' to catastrophic impacts​, at risk of Hothouse Earth due to ​slow feedback climate sensitivity ​
Global water & food insecurity & insufficiency
Increasing  risk of crop losses all best agricultural regions ​from heat, drought & floods
Worst ​climate catastrophes in the making

o Increasing ​extreme weather​
o Arctic multiple amplifying feedbacks 
oArctic switched from carbon sink to source ​
o Oceans-  warming ​+deoxygenation​​​ +acidification​
o Amazon & tropical rain forests switched from carbon sink to source​
o 6th Mass extinction ​​of life, to be boosted by global climate change 
Climate Emergency Institute 

May 2014 West Antarctic ice sheet likely
​in irreversible collapse.
Atmospheric CO2 highest in 4 million years (NOAA)
​1 June 2020
Atmospheric CO2 possibly 23 million-year high 
Increasing worldwide disastrous climate change impacts already, and far greater committed ​(locked in) climate change should make the emergency obvious.  
Atmospheric CO2 is record high and accelerating, CO2 is the highest and fastest increase rate in the past 15 million years

Ocean acidification is ​at 20 million year high and 300 million high for the rate  Jelle Bijma 2013
The emergency is now recognized in the UN Paris Agreement . ' Recognizing that climate change represents an urgent and potentially irreversible threat to human societies and the planet 
Also recognizing that deep reductions in global emissions will be required in order to achieve the ultimate objective of the Convention and emphasizing the need for urgency in addressing climate change' Paris Agreement Dec 2015

However the 'Intended' national emissions targets puts the world on track for 3.5C up ​to 4.6C by 2100, which is 8C after 2100 (full equilibrium warming) from Climate Interactive. This is certain catastrophe to the entire human population and almost all life (judging by past mass extinction events). World food production would collapse by 2050. 

SURVIVAL For the first time a July 2015 paper finds that our mismanagement of Nature puts our
​survival in jeopardy​​

A NASA based 2014 computer model report ​​warns of collapse of civilization
​industrial civilisation headed for 'irreversible collapse'?

A 2015 climate food production projection finds that '​​climate change
​ ‘set to fuel global food crisis, (and this report also includes
​ bee pollinator colony death crisis due to pesticide use and climate change). 

​The definite global emergency today is because of today's committed global
limate change, with all climate change indicators accelerating (StateofOurClimate.com),and no agreement to reverse global emissions
​(into decline)

Who does the emergency apply to?

As shown (click above link) the list has grown fast over the past decade.
Now it is almost all of us alive today, and certainly applies to all today's children everywhere and certainly
​to all future generations

​​The fast changing Arctic is an emergency for the future of the life sustaining biospshere and life.

Already committed (locked in) global climate change.
​1.5C ​by 2030-2040 and 2°C by today's atmospheric GHGs  ​
​(IPCC AR5 WG1 Ch. 12) see Constraints on Cumulative Carbon Emissions. 
As reported in IPCC AR5 WG2 Food   (see Box 7-1 | Projected Impacts for Crops and Livestock ...)
by a 2°C warming all crop yields in all food producing regions will be in decline, except only for temperate rice.
Summer high temperatures and extreme heat will become too high for NH temperate as well as tropical crops.​
OCEANS ​Atmospheric CO2 & GHG pollution are causing rapid ocean warming (now going deep) acidification and deoxygenation- a catastrophic triple assault. Warming + acidification is projected to lead to irreversible collapse, which would be disastrous for live on land.
The most important IPCC finding ‘Projected future climate change and other human-induced pressures are virtually certain to be unprecedented compared with the past several hundred millennia’. (IPCC AR4, WG2, Ch. 4.1 Key issues)

Atmospheric CO2 is at a 3-5 million year high and the CO2 rate of increase is unprecedented  'never before been seen' (WMO GHG Bulletin No. 13 Oct. 2017) 
o Rate of ​industrial CO2 emissions is unprecedented in​ the past 65 million years , which 14,000 X faster than CO2 can be removed.
o ​The terrestrial biosphere is now a net source of greenhouse gases to the​ atmosphere
o Tropical rainforests have switched from carbon sink to source ​
o The Arctic has switched (NOAA 2016 Arctic Report Card, and 2019) ​from carbon sink to source
o Ocean acidification is at a 20 million year and 300 million increase rate high, and 
"It (record of 2015 -16) emphasises that we have no carbon budget left for the 1.5°C target and the opportunity for holding to 2°C is rapidly fading unless the world starts ​cutting emissions hard right now". Prof M. Mann March 2016  (climate Code Red quote)
2019 NOAA Arctic Report CardArctic switched from carbon sink to source​. 'These observations signify that the feedback to accelerating climate change may already be underway'.
Sept 2017 Amazon switching from carbon sink to source
27 Nov 2019 Climate tipping points — too risky         MUST ACT NOW
2022 all climate change indicators are increasing as fast as ever, ​atmospheric GHGs faster than ever. Atmospheric methane is soaring at an explosive rate from warmed wetland methane feedback
UNEP 2021 Facts About the Climate Emergency 
5 Nov 2019 World Scientists’ Warning of a Climate Emergency William J Ripple et al 
28 July 2021 World Scientists’ Warning of a Climate Emergency 2021 (update) W. Ripple et al​
IPCC AR6 Multiple lines of evidence show that the rate at which CO2 has increased in the atmosphere during 1900–2019 is at least 10 times faster than at any other time during the last 800,000 years (high confidence), and 4–5 times faster than during the last 56 million years. IPCC AR6 WG1 Ch5 Exec Summary 
2023 Climate catastrophe fossil fueled business as usual continues

Big economy governments, big banks and fossil fuel corporations are still keeping the world on business as usual fossil fuelled expanding world economy, governments subsidizing the fossil fuel industry heavily.