The Arctic sea ice plays a critical role in regulating global warming by reducing the rate of Arctic warming by reflecting incoming solar radiation and also insulating the atmosphere from the warmer water below. Without this protective sea ice buffer, Arctic warming will intensify quickly, accelerating global warming. Thus, we can regard the Arctic sea ice as the immune system of the human body, which protects us from harmful things. Due to the increased greenhouse warming by human activities, Arctic sea ice has been rapidly melting during the past decades and the ice-free Arctic condition was expected to occur in September, when the Arctic sea ice area is smallest during a year, in the mid-21st century unless we reduce greenhouse gas emissions aggressively. However, there remains a large uncertainty in this prediction and our recent updated study has found that ice-free Arctic conditions can arrive as early as the 2030s. This is faster by around a decade than previously reported by the recent Intergovernmental Panel on Climate Change (IPCC). Moreover, our results show that an ice-free arctic will be there in summer irrespective of our effort to reduce emissions, which was not expected. This means that we need to prepare adaptation measures for the faster Arctic warming and associated impacts. Since Arctic warming is suggested to bring weather extremes like heatwaves, wildfires, and floods to northern mid- and high latitudes, the earlier onset of an ice-free Arctic also implies that we will be experiencing extreme events faster than predicted. More importantly, an ice-free Arctic is predicted to occur even in a low-emission scenario around the 2050s, indicating that we need to reduce CO2 emissions more aggressively. The low emission scenario we considered is the Shared Socioeconomic Pathways (SSP) 1-2.6 and its path assumes net zero emissions achieved in the 2070s. To keep Arctic sea ice during summer and thus avoid associated impacts, we need to achieve net zero emissions earlier say the 2050s, which is equivalent to achieving the 1.5 degree C target of the Paris Agreement. 
  This projection can be described as a tipping point at least in September, but the stronger reduction of emissions would delay the arrival of an ice-free Arctic and more importantly, avoid the expansion of the ice-free Arctic into other summer months. Compared to the low emission scenario, higher emission scenarios project ice-free August and October before the 2080s, indicating that higher emissions will surely bring stronger impacts. The current policies based on nationally determined contributions (NDCs) will be close to the SSP2-4.5 scenario (intermediate emission path) which projects about 2.7 degrees of global warming relative to the preindustrial condition. In this scenario, our results indicate that the first ice-free years will be experienced as early as the 2040s in September, also expanding into August and October before the 2070s. However, if we can achieve the 1.5-degree warming target, we can see Arctic sea ice remain in most of the months with overall some reduced area compared to the current conditions. This implies that Arctic sea ice can be recovered under stronger mitigation policies since the Arctic sea ice area is closely related to global warming levels and cumulative CO2 emissions.
  To reduce errors and improve predictability, we have adjusted future projections of the Arctic sea ice area using a rigorous method of observation-climate model comparison. From this comparison, we have found the dominant contribution of greenhouse gas increases to the observed sea ice reduction during the past four decades but also found that models on average underestimate the observed trend. We have applied this information of model underestimation and constrained the models’ future projections. Global climate models we used are recent versions of computer programs that can simulate Earth’s climate system based on physical laws considering atmosphere-ocean-sea ice interactions at 1–2-degree resolution. To find the causes of the observed Arctic sea-ice changes, we used climate model simulations performed under different historical forcings including greenhouse gas only, anthropogenic aerosol only, and solar and volcanic activities. After estimating the response patterns of the Arctic sea ice area to each forcing, we related these ‘fingerprint’ patterns to the observations using a multiple linear regression approach. This technique enables the breakdown of observed trends into each forcing contribution, and we found that greenhouse gas increases explain most of the observed sea ice reduction throughout the year whereas other forcing factors exert much weaker influences. This procedure is important for reliable future projections which are basically determined by greenhouse gas emission scenarios.
  An ice-free Arctic will have not only regional impacts but also global implications. Upon ice-free conditions, animals depending on sea ice will be affected more strongly. As Arctic warming is enhanced, permafrost will also melt earlier and many species including polar bears, walruses, and reindeer, will have trouble surviving. Arctic indigenous communities will also be affected a lot by their hunting and traveling. Reduced sea ice will open the routes for ships and favor the development of natural resources. This can be beneficial by reducing the shipping industry’s carbon footprint. However, Arctic warming and melting permafrost can enhance the release of greenhouse gases into the atmosphere, intensifying global warming. Ice-free Arctic will induce faster and stronger Arctic warming and Arctic warming may increase extreme weather events. Based on some recent studies, stronger Arctic warming, so-called Arctic amplification, is expected to induce changes in atmospheric circulations like more fluctuating jet streams in the Northern Hemisphere, potentially increasing chances of heatwaves, wildfires, and heavy rainfall events over the Northern mid-latitude lands including North America, Europe, and Asia. It is imperative to assess the possible influences of the faster ice-free Arctic on human society and the ecosystem, for which urgent collaborative research is required.